Basically, T-Mobile UK uses exactly the same APN (general.t-mobile.uk) for both general internet browsing and handling MMS messages. The only difference is the proxy, which is not used for general browsing, but is required for MMS. Since fMMS always uses proxy, regardless if "Use proxy" checkbox is set or not, it's quite easy to use the same APN profile for both internet and MMS - add proxy IP address and port to the profile but keep "Use proxy" checkbox un-ticked. :)

When I've had a separate "T-Mobile MMS" profile set in device, some apps like browser or email client sometimes tried to use it to connect to the internet, which obviously didn't work, as proxy did not pass non-MMS traffic through. I've had to manually reconnect with "T-Mobile Internet" profile, which obviously wasn't very convenient at all.

Anyhow, I generally think it's NOT right way of handling this in Maemo 5. What I'd rather love to see is native way of configuring multiple GPRS/3G connection profiles (by native I mean via regular system's control panel, not 3rd party apps like fAPN) with some sort of categorisation/priority, ie. where I could set that profile ABC is default profile for general browsing and XYZ profile is just for MMS.