This probably known, but just in case it hasn't been reported here it is.
I was testing with the new particles - the FPS was around 30. Then i was trying to delete something when the powder toy didn't freeze, but instead wouldn't let you place or delete particles. Everything else was working fine. I have windows vista if it helps.
this has been noticed by some windows users, but it doesn't seem to be a powder toy problem, try going to another window and clicking there then going back to tpt.