There are apparently three operators in Greece offering mobile broadband: Cosmote, Vodafone and Wind.
Vodafone offers a PAYG sim called KartoInternet
The €20 for the SIM include 15 days of internet, further prices are listed on the webpage. During summer '12, they offer it at €15. It supports Skype but can not be used for normal tlephone calls.
Ortwin Gentz at FutureTap describes how to get 3G data access using Vodafone KartoInternet.
Surf & Email 60/100 data packages.
There are two additional packages for telephone & data called Surf & Email60 and Surf & Email100, good for 60MB (@3€) and 100MB (@5€) respectively. They are intended for smart phones and can be purchased by an activation call or SMS from a prepaid Vodafone CU SIM card. The charge is deducted from the previously loaded units.
(Attention: In August 2012 there is a promotion to get 500MB for 5€ istead of 100MB on the Surf & Email 100 package which makes it a serious alternative to the 20€/15day kartointernet package if you do not need excessive volume.
Attention: they seem to be degrading Skype throughput so it is virtually impossible to Skype with these packages!
Attention: if you load 5 Euros, you actually get 4,86 Euros net worth because of a 12% telecommunication tax.)
They can be used in a USB data stick as well using the "webonly.vodafone.gr" APN instad of "web.session" (this one is for the 20€/15day plan). Both need different SIMs, but the CU (prepaid telephone) SIM is free of charge. To purchase anything mobile in Greece you need to register with proof of identity (passport).
Both plans are good for 30 days (The data has to be used up insiede a month from activation) and can be activated by calling 1286 and typing code "55" for S&E 100, or send an SMS with SURF60 or SURF100 to 19109 (you need to put the SIM into a phone to do that).
Wind sells two data plans under the F2G banner with high data caps. It seems possible to add voice minutes to these too, but I didn't try.
F2G ADSM Non-Stop 7: 1 week PAYG data plan with a 12GB (yes, gigabyte) cap for €14
F2G ADSM Non-Stop 2: 2 day PAYG data plan with a 3GB cap for €7
(I got solid 3.5G coverage everywhere, even in buildings, from Athens to the far side of the island of Syros)
WIND does not advertise these plans any more but they are still available (May 2012). You can activate the plans with a SMS to 19595 with the text 'NS7' for 7 days or 'NS2' for 2 days.
Wind/Q (same provider) you can get 500MB for 30 days for 5 Euro (does not renew) by purchasing a prepaid card (1 Euro and you can add any amount you wish at a Wind store) and texting:
To 1218. They give you 500 MB and confirm it in the reply SMS, don't worry about texting 100MB. It's the way to go. In the summer they only give you 200MB.