Given the data received from sender is merely a MIME encoded SMS pointing to URI to download message if you have the same APN all you have to do [as proof of concept] is decode the SMS and download the data from the URI. URI can be WAP 2.1 or HTTP. Then you need to decode that data. For convenience all of this must be automated for end-users, and a second PPP connection must be made only if APN are not same IOW this should be checked. I'm not sure most providers require a seperate APN. I find this a bold statement which requires a lot of recent experience with many international telcos while this might very well be a legacy statement.

If providers have a list of smartphones which supposedly support MMS then this might destroy the opportunity for community-supported MMS.

Indeed Nokia has the inside knowledge of MMS support, but those who implemented it in S60 did this long time ago and are not from Maemo dept. I think if Nokia would do it then need to implement full WAP because sometimes WAP is required instead of HTTP and they can't implement a half-working WAP since they have to respect standards. If community supports MMS it may or may not work completely, and it may or may not completely respect respect standards but if Nokia does it they have to completely respect standards.

The whole Brainstorm entry is about whether Nokia or community would do it, but I don't think this proposition is the only question to be asked. IMO the question should be where the priority of minimal support lies to make most users be able to use Nokia N900 as their phone. My answer to that is: a community project to provide support to receive and read a MMS. As quickly as possible. If 5% of providers need seperate APN I don't find such support very important, while if 95% of providers need seperate APN I do find that important. Also, is it possible to embed MicroB? Then you could use the WAP plugin to download the MMS.