I am not currently on T-Mobile. I'm currently on MetroPCS -- no contract, but also lousy data service. I'm planning to switch to T-Mobile in the near future. Why I want T-Mobile is: best data plan prices. Their 3G network is WCDMA 1700, thus, I want my 3G device to have WCDMA 1700 support.

The only IM client that I think you could say is a foregone conclusion is GoogleTalk. And for Mail, they could leave out an IMAP client and assume you'll access mail through google's mobile Mail app and/or through the web browser to gmail.