So far, I have restricted myself to building applications against the 4.1 API to ensure backwards compatibility with the 7xxx series of devices.

The BlackBerry 4.2 API provides a number of features that make life a lot easier and will just enable better applications. The subsequent API versions add a few more things that help too.

Does anyone have any thoughts, or better, real data about the number of 7xxx users vx 8xxx users, and the likelihood of a 7xxx user purchasing 3rd party software vs the likelihood of a 8xxx user doing the same ?

This information would be really useful to help decide which API to build against.