I've read on the website of greek mobile operator that he offer a pre-paid internet access. So if your PCMCI card support GPRS/EDGE it will be suitable. So you need to buy in Greece starter pack with SIM card only (and to charge account from time to time). Make sure of your PCMCI modem has not SIMLOCK (that other operator SIM card will be work properly)
Unfortunatelly I'm forgot the name of the operator :| but part of his website was in english so you sure find it via google.
:: Marcin Jablonski