Mmm, I started getting this too. I'm fine for about 50 miles or so, but once that hits, I really start feeling it. I've got some Butt Butt'r but I use it rarely (mostly because of the feel of it) though I'll probably get more into the habit now that my rides will be lasting longer.

For mild irritation, I like a little Gold Bond powder. This weekend, however, I've got two longer rides ahead of me, so I'll be using the Butt Butt'r.