So I found this

http://forum.xda-developers.com/arch.../t-549238.html

It does seem to be a t mobile server side issue, however in the link above, they have been able to change an andriod hand set mms user agent to spoof the sever and get full size mms.

So it seems that if someone could find a way change the user agent to a n900 it should work.

edit: I pulled this out of fmmswappush.py

{'User-Agent' : self.config.get_useragent(), 'x-wap-profile' : 'http://mms.frals.se/n900.rdf'}

I did a little more research there are several different mms architectures ...Im going to guess that if the n900 arch and the n9 are different this probably wont work. If this is infact the case you will have to find a phone that uses the same arch and is supported by t mobile US.