I just found this thread, after posting in a few other threads. I had some questions regarding email that I have mostly gotten answered, so I'll post a summary here. Some of this might make sense to add to the FAQ up top.
I believe I've determined that:
1) Exchange email is a separate App from other email (i.e. no unified inbox with Exchange + non-Exchange accounts combined)
2) The HTC mail app does not provide push delivery for Gmail accounts, use the Gmail app instead.
3) Google Apps Domain accounts can be used in the Gmail App, with the same performance/features as Gmail accounts.
4) The GMail App can support multiple Gmail / Google Apps Domain accounts, but only the "primary" account will sync email/contacts/calendar. The secondary account(s) sync only email/contacts.
A few still outstanding questions are:
5) Is there a way to see a single calendar view that shows both Exchange and Gmail calendars simultaneously? There were some indications that maybe third party apps might support this (Pure?).
6) In the Gmail App, if I have multiple Gmail / Google Apps Domain accounts, do I see all those accounts in a unified inbox? Or do I have to manually switch between them?
I have to say, it's a little bit frustrating that the Android platform doesn't do a great job of managing various accounts (including something as obvious as multiple Gmail / Google Apps accounts). You'd think that would be a no-brainer for Google to get right. I'm currently using a BB, so I'm used to the unified view (both email and calendar), and it's definitely how I'd like to use the device - I don't want to be navigating around all over the place to check email and manage my calendar. I'm basically looking at Android vs. iPhone4, and trying to figure out which way I want to go with my next device...