What version of JBoss Tools do you have installed?
That was a silly question, for Eclipse Ganymede you have to be using 3.0.0 Beta1 or CR1 (or maybe a nightly build)... those are still unstable but I am testing CR1 here and
intellisensedoes work. Are you using perhaps a nightly build?
Or perhaps you are not using JBoss Tools at all? (You need it to have
intellisensefor you Seam components).