Just for info for anyone visiting Cham this season, Summer 2013, there has been a considerable "change of heart" toward biking in the valley.
You no doubt already know about the July-August bans across the valley, however things have changed even more for this season.
Lift tickets for biking (Cham'VTT, EUR 17/day) are now limited to Les Houches and Le Tour (Col de Balme). You will not be able to purchase a VTT pass to ride at Flegere, and likely elsewhere - they will only sell you a 1-trip pedestrian ticket.
If you want to use mid-valley lifts, it may be advisable to look into the Chamonix Multipass ticket (for more than 4 days this seems a reasonable price in comparison to the Cham'VTT pass). There is also the Chamonix Rapid Card which has a one time initial fee and then costs EUR 23 Euros per day you use it, and allows access to every lift in the valley.
Flegere DH course is no longer maintained, and most of the woodwork sections have been removed (beware the rusty nails and nasty holes in the ground). The Compagnie de Mont Blanc no longer wish to encourage bikers here and in the middle of the valley, hence their decision to abandon the DH course.
Vallorcine DH is closed for the season due to works ("travaux"), and bikes are not allowed on the public transport between Switzerland and Argentiere because they have switched the trains for buses. http://www.chamonix.com/pdf/horaires-train-travaux-ete-2013.pdf