pkb:But T-mobile wouldn't allow tethering - at least the option didn't seem enabled in my settings. I didn't look into it at all though.
They do allow tethering, with restrictions (of course):
T-Mobile Smartphone Hotspot/Tethering T&Cs
It's a lot more visitor/tourist friendly than the AT&T offering.
I'd be tethering a WiFi only iPad. At this point I'm just weighing things up. There'll be WiFi access in the hotel room(s), and I doubt we'll be lugging the iPad out and about that much anyway.
However, if the WiFi is rubbish, then tethering is a nice backup option if needed.