I have the Matrix, not the Duo, so mine doens't have Window's Mobile. (it was free with rebate when I got it last month but I think they changed the promo) That doesn't seem to be preventing me from doing anything I want to do. I think I can't add the gmail application but I can still get to my mail from iGoogle.
We have AT&Ts media bundle with unlimited text, internet & messages. It's $30 a month. Data unlimited is just the web, no texts or IMs and it's $15