Ive been doing some playing around, seems as other phones accept my .3gp vids if i send some text with it. Ofc this will prolly depend on the reciver phone atleast it worked with 5800 and n85.

mencoder currently doesnt support amr(3gp sound standard) out of the box but i stumbled over a page with amr install instructions
so i dropped the maintainer of mplayer(n900) a line about it, ill keep you updated if i get an answer