I am about to go to Greece, so I haven't tried this yet, but "Wind" claims to provide unlimited GPRS & 3G for €3.49 per month

http://www.windplus.gr/default.asp?siteID=2&pageid=2&langid=2

For the first time in Greece! Unlimited WAP browsing for WIND Plus WAP portal and for any other WAP site in the world through the WIND Plus Non-Stop program! WIND Plus Non-Stop offers unlimited browsing for just 3, 49€ per month (VAT included)! The service can be activated by sending a free of charge SMS with the text “PLUS” to 3490 (for post-paid subscribers) and to 349 (for prepaid subscribers) or by calling 1260 (€0,24/ per call).In addition, when buying selected 3G mobile phones you are granted for a period of 3 months, free WIND Plus browsing, while when subscribing to the WIND Max and WIND Platinum programs, WAP browsing is completely free, for as long as your subscription stays active!

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Another Info: This looks interesting (not checked):
http://www.wind.com.gr/pages.fds?langID=2&pageid=1443
Wind-Prepaid, 10€ for 2 Days unlimited Internet Access
30€ for 7 Days unlimited Internet Access

Coverage of Wind: Coverage Map