No-one will allow a mobile to access a contract-ISP account
Most half decent ISPs (so, not the Tiscali-type crowd) will give you a number which can be dialled over GSM, so you can usually find a way to dial it from abroad too. It won't be a free call from abroad, and I guess there may be a problem with dialling free numbers over GSM anyway (never really thought about that; usually it isn't allowed).
Dialling up one's ISP directly is a very versatile old tried and tested way of getting internet access worldwide. Pricey and slow but when it works, it works and there won't be any network-controlled port blocking because the network doesn't see the protocol. For all it cares, you could be sending a fax.
GPRS or 3G is the way to go forward, but the GSM network provider (the one who the SIM card came from) is now your ISP, so you need access to an authenticating SMTP server if you want to transmit emails. I have one set up on a server at the office, and there are commercial outfits (and decent business ISPs) that will provide these too.
The T-Mobile GPRS deal is, I suspect, limited to the UK. You can get bundles which are usable abroad but only on more pricey contracts.
It's the old problem: PAYG or contract.... unless you are firmly at one or other end of the data usage scale, there is no easy solution.