There are some years that the Sierras will get significant snowfall in early October, so you may need to be flexible. The road through Tuolomne will close for decent snow, but often melt out completely within a day or two later. Nights will be cold. October is usually perfect in the Valley but watch the weather, as tremendous storms can occur. Places like Donner's Pass and the Leap will likely be spectacular as well.
I miss October...
I want to second Chip -- late September is our cutoff for Yosemite. Any later and weather becomes too much of a roll-of-the-dice for 3,000 miles of travel, etc. Obviously the earlier in October the better. Hell, starting in October the Adirondacks start to see snow.