In reply to llanberis36:
I climbed it a number of years ago in the second week of Sep in perfect weather. It was my first alpine route. We had been unsuccessful in an attempt on the Monch in the previous week due to bad weather. Not wasted time as it helped with acclimatization (we spent a night at altitude).
We aimed to go up by the Gouter route and down by the Grands Mulets route. The Gouter route was in good condition, but the Grands Mulets route was not in condition with thin snow covering the crevasses, so we ended up climbing back up and descending by the Gouter route (knackering!). It might be that the GM route is not a good option late in the season - probably worth investigating. You can check current conditions of the routes with the guides office in Chamonix. They are very helpful.
If you're thinking of using the telepheriques, you may want to check what dates they stop. We went to Chamonix for rock climbing in mid-September (on the 18th I think) a couple of years back and found that nearly all of the lifts had stopped for the season on that day! Having said that walking up from the valley is a nice way to climb Mont Blanc.
The Aiguille de Midi lift runs later than the others but is expensive!
Hope some of this is helpful!
Cheers,
Chris