1) There's basic IMAP support, but no IMAP synchronization. Try getting BES Hosting service ($15 per month) if you want 100% wireless synchronization between your Outlook and your BlackBerry. It will be Exchange instead of IMAP, but you will have your BlackBerry mirror Outlook in realtime (including Email, Tasks, Notes, Calendar, Addressbook). Doesn't matter which BlackBerry.
2) T-Mobile has a slightly more full featured one, but Cingular will probably eventually catch up. But if you gest BES Hosting service, this is moot. For more info about BES hosting, see my FAQ in my signature.
3) Web is much faster on the 8700 than the 7105
4) Both can be used as a modem. If all else fails, you can use Shark modem from
www.mobishark.com if the builtin modem is too limited.
5) Both use the same quad band GSM technology, however, Tmobile and Cingular has different roaming agreements. However if you unlock the device, you can use a 7105 on Cingular, and an 8700 on T-Mobile. Unlocking may cost extra, though. (Tmobile will carry the 8700 after Cingular's 90 day exclusivity is up on the 8700, but you can get an 8700c or 8700r unlocked for Tmobile if necessary)
6) You can have wireless realtime OTA sync of PIM data -- just get BES hosting. Yes, it's subscription based. It's very convenient. See my FAQ.
7) 8700 has a much faster CPU, and also thus faster web browsing. The 8700 has EDGE, and Cingular has a bigger network than TMobile (But not a superset of TMobile, so TMobile serves certain areas better -- check your reception in your area)
8 ) You can unlock the 8700 and use the 8700 on T-Mobile. Alternatively, wait until March or April for the genuine T-Mobile 8700.