You're apps should all be restored from the market once you set everything back up when you flash. Other options would be to back them up with something like Titanium Backup or My backup... they are in the market.
That is a good question but I'm not 100% sure. Reasonably speaking I would think so as long as its a rom of the same platform. Given the issues going between 2.2 and 2.3 I'd say not that way. But 2.2 to 2.2 or 2.3 to 2.3 it may. I personally for not use that feature to say for sure but I can check with a couple of people that probably would know. I will definitely let you know what I find out though.
Just so you know... I looked into your question on apps on the sd card and switching roms. The answer I got was the same as what I had thought as well. We would think the apps should be found and recognized but would still suggest making a backup with Ti or your backup app of choice just in case.