Data Plans

[RTuesday]

GT-sim (see the threads on here) has gprs roaming to some countries at Euro 1c/kB (US$0.012/kB, I don't know how that compares with your T-mobile rate). They say the countries they cover so far (more "coming soon") are:

GPRS RoamPartner for GT-SIM Cards

Base Belgium
Rogers Wireless Canada
Bouygues France
Orange France
02 DE Germany
D2 Vodafone Germany
Cosmote Greece
Cable & Wireless Guernsey UK
HK CSL Hong Kong
O2 Ireland
TIM Italy
Jersey Telecom Jersey (UK)
P&T Luxembourg
Optimus Portugal
Singtel Singapore
MTN South Africa
Telefonica Spain
Vodafone (Airtel) Spain
Sunrise Switzerland
Etisalat UEA
O2 UK
Orange UK
T-Mobile UK
Vodafone UK
AT&T USA
T-Mobile USA

I've also used it for GPRS on Vodaphone Ireland and Manx Telecom, so that list is not complete.

Another option is a Virgin Mobile SIM from the UK, that has good GPRS roaming (more than the above) at a flat global rate of 0.5p/kB (US$0.00875/kB).