It's been happening for a while - but not from day-1. Unfortunately, the frequency is so sporadic, that it's hard to tie it back to any particular change. It's rare for it to happen more than a couple times a week, and it may go a couple weeks between happening (this is with 2-4 hours of use per day).
I mostly notice it when switching in/out of a couple of GameHouse.com games -- but I'm *pretty* sure I recall it happening before installing them. I think I just notice it with them because they're the only apps with sound that I use with any regularity.