Alain,
the best way to accomplish this I think is via the use of the environment variable TM_LATEX_MASTER. If you have a project, you can set an environment variable just for the project with name TM_LATEX_MASTER and with value the full path to the file you want compiled. Then, no matter which file you work on, the file pointed to by TM_LATEX_MASTER will be compiled instead. The great thing is that if you use pdfsync, that still works as you'd like it to.
Have you seen the LaTeX in TextMate screencasts? I think I've described that there.
Haris