[Schlips1]

That's nice because I asked Vodafone Gr and got the answer:

"3. You are not able to register your Prepaid from outside the country.
According to the New Law (N.3783/09) for All mobile numbers (Registered
before the 09/11/2009 or New ones) must be registered under your
Personal details. Otherwise your mobile number will be inactive on the
31/07/2010. You have the time to do so until the 30th of July 2010.
After that date if your number is still unregistered then it will be
immediately deactivated without any further notice. So, you must visit a
Vodafone shop here in Greece to register your Prepaid connection. There
is no any other procedure from abroad and until the registration you are
not able to use it. E-mail or web registration is not possible.
All that you need is your Passport and the sim card of that mobile
number. If you please pass this Important information to your friends if
they have under their possession a Greek Prepaid mobile number
unregistered, no matter from which Network (Vodafone or other). The
registration is free of charge."

Vodafone has been the only one, which put in "Hotel" at an address during registration.....