Hiya,
I've been fiddling with videomodes for a while too as well, and looking at the ps3fb code. Note that the git kernel sources obtainable from the link at
[Please register to see this link. ] include a 1366x768 mode in the ps3fb code ( or was it 1368, can't remember). Anyway, I tried forcing X to use this resolution but have had no luck sofar :-}
Compiling a kernel on my PS3 takes about 30 minutes, so it's not too long actually. Not worth the hassle setting up a crosscompiler if you ask me (tho it is cool ofcourse
). I used the instructions at the link mentioned, but without the crosscompiling stuff. Once you have your kernel configured you can also use genkernel to build future kernels.