Just a little reminder:

Many network providers, on seeing their SIM in a new phone, send a special SMS containing settings for that network. For example, 3 UK does this. Some networks will send the special SMS when you ask an operator to do it, or when you first text a number to trigger it.

It looks like the N900 can process the special SMS, because when I put my 3 SIM in and booted, the GPRS settings were correct already, but then I receieved two regular texts from 3 telling me they were about to send me settings, and shortly after, I heard an alert tone with a message on-screen saying the new settings did not need to be applied because they had not changed.

Anyways, the point is they send full MMS settings as well as GPRS settings in the same special SMS.

It would be really worthwhile figuring out how to use the MMS settings that have been sent. Perhaps the N900 stores them when it updates the GPRS, even though it doesn't use the MMS settings natively yet? If not, perhaps that can be changed?

Note that it isn't possible to work out the MMS settings just from the SIM info on all networks, but the special SMS should always work, if they send it. Not all networks do send it, though.