We agree that the ability to easily move between fields on forms is a major time-saver. Unfortunately, Android gives control over that behavior to individual apps. Ie. the apps actually tell the keyboard to put "next" or "go" or "search" in place of the "enter" key.
Presumably, though, Keypurr behaves the same as the standard keyboard to give equivalent operation.
Although there is no Tab on the standard Android soft keyboard, there is one on the hard flip-out keyboard on my Desire-Z. This works as you'd expect a tab key to do, moving between fields on forms and inserting a tab in text, according to the app.
It's clear that Android supports a tab-key function, therefore any alternate keyboard driver should be able to provide the required input. I don't think you should try to do anything clever with the Enter key - making it do different things at different times would be a minefield!
I think the challenge for Keypurr is where to put a tab key, as every tap and flick space is already used! Well all, that is, apart from "Cursor:". This seems to me a bit of a wasted space. The cursor symbols would be better as proper arrows, rather than chevrons, making them self-explanatory, which would render the "Cursor:" label redundant.
I don't think this would be a good place for Tab, but would allow things to be shuffled around a bit. You could put Fkey on/off here instead of shift+flick, making one more space on the main keyboard, and perhaps do a bit more shuffling.
Food for thought?
Rick