Oh man, I have to copy my response off of HoFo - I just answered you there!
The T-Mobile pay as you go card will not offer you BlackBerry Service. Your account (T-Mobile US or one of the Greek carriers) must have a BlackBerry Data Plan on it. Without the BlackBerry Data Plan, you will have voice and SMS, but not data, Internet or e-mail.
If the Greek carriers you mentioned do not sell the BlackBerry plans, you'll have to find out if any of the US carriers have BlackBerry roaming agreements with the Greek carriers and roam ($$$) there.