Various comments for future development, some of which are related to WA/CAS and some to ND1 more generally:
When entering expressions, adding single quotes is a real pain, especially around variables. Sometimes one can do it without quotes and other times it is required. I know you have mentioned working on this, but if it is possible to eliminate the need to require quotation in expressions, that would be great! Having equations potentially work both ways is confusing. Users shouldn't have to guess/remember where it works and where it doesn't.
A related issue is having to enter text by first tapping the "rpn:" input line; this takes an extra step for such a common use case. It would be nice for the single quote key to automatically launch the keyboard. Not sure what that breaks for other uses. Also, similar functionality for [, ( keys would be useful. Also, havering an analogous " key would be nice.
On the topic of keyboard, I know that Apple frowns on different keyboards by default, but what about a customized ND1 keyboard for editing within ND1? It's really awkward to enter mathematical formulas; lots of shifting back and forth to enter symbols, digits and operators (also delimiters). Digits and operators on different keyboard screen is particularly bad.
Lacking a custom keyboard, I would hope ND1 1.5 will use new style of iOS 7 keyboard. If you can make a customized one, please make it have the iOS 7 style.
When WA is available, I would suggest adding several WA menus. One new Menu might allow for easy adhoc creation/editing of WA functions/commands. For example a WA-opt key would allow for input a vector of options. A WA, key might allow for executing a simple command [with options] on stack without having to create expression from scratch, i.e., eliminate << command WA>> syntax.
Another WA type menu would put the most common WA functions into their own WA/CAS menu, including: Simplify, Evaluate, Solve, Integrate, Differentiate. By putting all of these under one menu, it will create a much better user experience. Inside these Menus, users will know that network connectivity is required and that there will be a 1-3 second latency. Outside of the WA menu's, command execution will execute locally and quickly. This really should provide for a better user experience. Moreover, if the WA functionality is an extra charge, it's easier to isolate that functionality to several menus than having different default keyboards--or worse the same keyboard with non-functioning keys for users not paying for WA. I wouldn't put anything on the main keys that makes WA calls. Personally, I have found the WA latency to be acceptable, albeit it barely. (Anything you could do for speed would be great, but probably not in your control.) The issue is expectations management. I should know when and when not to expect a command to execute remotely through W|A.
Related to the prior point, I'm not sure that Integration and Differentiation (even if implemented locally) belong on the main keyboard. My guess is that they are much less used than many other functions, including for example a numerical pi, trig and/or log functions.
For 5s users (or retinal display users) is it possible to add soft-2nd-level-keys? This would be a huge addition! (If screen space is a problem on the non-retinal display phones, make it a configuration option.) Since you have suggested getting rid of the 2nd-level key in favor of holding the primary key longer, it would be great to have the same functionality for the soft-keys. That would provide for a huge increase in primary keyboard functionality, reducing the frequency of dropping into the Menus.
I know that holding soft keys brings up the Help system. I would not get rid of the Help system. Instead, I would implement it with a special 'Help' key that one could execute prior to executing the key one is getting help for. This has the added benefit that you could provide better Help support for all keys, not just soft keys. (Also a big win.)
By the way, getting rid of the 2nd-key in favor of just holding the key is a great idea. I haven't used the 2nd-level key since its been provided. Its much faster to hold a key slightly longer in order to access the secondary functionality. Also, picking up an extra key is nice. The only thing I would try to do is to continue to have some sort of cue that the 2nd-level functionality worked. Right now, it's by lighting up the 2nd-level key itself. Perhaps the cue would be changing the color of the label on the secondary function.
As you begin to fully flush out the CAS elements of ND1, I would start with the view that a 'Naive' user will want the main calculator keyset to act like a high end rpn calculator. CAS should be there, a step away, not in the way.
Moreover, personally, I would like to only use one calculator, the ND1. Currently, I still use the HP 42s because very common calculations involving numeric pi, trig, log and/or scientific notation take more keystrokes on the ND1. Consider designing the ND1 keyboard modeled after the HP 42s (the iPhone app is gorgeous in look and engineering design) rather than trying to copy the HP 50g (for which there are nothing but hideous iPhone apps.) The former is much more consistent with the form factor of a mobile device than a squeezed down HP 50g.
Finally, and please don't take any offense, but how about a more descriptive name for the product? ND1 is very obscure. I literally have tried every rpn calculator or high-end mathematical app for several years and didn't even come across ND1 until recently, and then only by chance.
With the programming hooks built into ND1, it is a very powerful and differentiated product. The ability to drop into RPL or Javsacript provides unprecedented power, but even programmers don't want to work when they are doing the obvious. Mac OSx should be an inspiration. It has the GUI and easy of use that appeals to my parents, but a unix underbelly that is there for the power user/programmer. ND1 offers the same opportunity. Please keep up the great work!