I've gone for a Blackberry Pearl (after all, you all know how much I liked it when I reviewed it), and have chosen a T-Mobile Flext package, with added Web'n'Walk, as well as Blackberry email. I'll hook up T-Mobile's BIS to point at my mail server, as I know that all works well - I've been reviewing some other Blackberry kit on T-mobile recently.
When the phone arrives tomorrow I'll give them my PAC, and should stay on my current number.
One thing I need to check up on (as I don't quite believe it), is the cost of Blackberry data when roaming. This page seems to imply that it's only 0.73p/kB when in the US. Which makes leaving email on when I'm travelling quite affordable.
Thanks for the advice. AKICILJ indeed.