No but I have been very tempted to.
There's no formal rule that we can't but it is frowned upon if we are doing things on the Internet that aren't related to our professions.
Alot of the time at work, it's slow and boring with alot of dead periods in between customers. We are caught up on what needs to be done. So on some days I have alot of potential time to check out the forum at work yet I don't because I don't want any of my colleagues to know I post here. I've posted some personal stuff and occasionally ranted about my job. If a colleague found that out, it could work against me.
So instead, I end up surfing websites and reading listservs related to my professoin.
If I had my own private office with a door (which I don't) I might check the forum now and then. Since I don't have a private office, someone could just walk up to me anytime and say "What you are looking at?"