Nope, last of August is not too late. In fact, my favorite time of the year is September, but you can get a cold front push through and provide some cold and perhaps even a little snow at the highest elevations, but if it does this the stuff will not last long, so you just hold up for a day or two and then enjoy (again, in late September) some beautiful Fall colors with snow on the tops of the mountains -- that is about as beautiful as it can get.
So, August no problems. In Glacier you can get an early snow at the top of the pass, but again, it should not last long and is in fact unlikely. There will be cold nights at the higher elevations in WY, MT, CO, and ID even in late August. I generally find WY to provide some of the coldest temps at night.
Southern Utah is actually much better in the 2nd half of September, but it is good anytime. Check out the link below to go to my suggested rides in Colorado and Utah. And, if you'd like to meet some folks from this board, check out the possiblity of coming to Rockies-Gold in Montrose, CO, 25-29 July.
As for South Dakota, I'm going to an FJR gathering in Custer, SD (Black Hills) around 20 September -- definitley not too late even then.
If any specific questions send me a PM.
Rockies-Gold:
http://rockies-gold.net
10-days of Riding in CO and UT:
http://coloradodave.net/CO-UT Ride/CO-UTride.htm