The main team that's working on the PS3 version isn't the one doing the port for the 360 version, if I recall. They'll get another team to do it or contract an outsider to do it by giving them the SDK's.
While I know Tri-Ace is currenty working on Star-Ocean 4 which should be out worldwide spring 2009 they could end up being the ones helping with the porting of FFXIII. They already have a connection to the company and they are the ones most familiar with the hardware considering that by the time FFXIII is released in Japan they will have already created 2 titles for the 360. This is of course speculation but it really isn't that different from much of the speculation surrounding why FFXIII is now a multiplat title.