Sport 2000 (formerly Twinner, next to McDonalds) in Bourg Saint Maurice were pretty decent for bike hire last summer. Kit was all Mondraker stuff, everything from cheap hard-tails to full-on DH rigs. From memory, pricing started from about 140 euros per week for a decent XCish Full-sus. DH kit very much more expensive. Definitely best to pre-book.
Some more info (all based on last year for now) on our site: Bourg Saint Maurice bike hire
We checked around pretty thoroughly and we reckon they were the best shop in town (used them for hire bikes for some of our guests). Vallon Blanc in Tignes were good too, especially for DH/freeride kit. La Grange au Skis in La Rosiere also excellent (Kona Coilairs and Stinkys) plus Mitch is a top bloke who knows the local riding very well.