Also note that a lot of "GPRS" access isn't open internet. It is often just WAP, and a lot of it allows access only to the network provider's POP server for email so basically useless when on the move.
The one PAYG GPRS network I know is wide open is Voda, but it's expensive and when roaming abroad there is still no knowing what limitations one will get; e.g. in the channel islands on C&W one gets WAP only.
There's a long way to go before we have a really reliable solution to all this that works through Europe, and that's forgetting 3G completely (poor coverage).
The other basic thing is that one cannot do useful WWW browsing (for weather charts and notams) with less than 640x480 and that rules out most PDAs.