I can understand your frustration.  Though I'd like to add a situation that frustrates me as well.  If I've got an important work related issue to discuss with one of two admins gossiping or discussing personal issues, I find it extremely irritating that I'm not acknoledged in a timely fashion.  I don't consider it rude of me interrupting.  Am I incorrect?  Normally I'd say something like "I'm sorry to interrupt, but...".  Especially if I've  traveled all the way from another floor to discuss this work related issue.  I'm curious if everyone considers this rude.