I actually thought I saw something earlier this morning... Saw a PR3 jump to PR5, and several others jump 1-2 points. Shortly after, it was all back to normal, almost like it was temporary, but I don't know how they would do a full TBPR push and draw it back like that.
I even submitted a new post here at WebmasterWorld, but asked the admins to retract it, after I couldn't replicate it.
There is no regular schedule. On average it's about four times a year, so three months apart. But that's an overall average and individual PR updates aren't predictable. Sometimes it can be 5-6 months apart and other times they just get packed up.