Dear Blonde Guy,
There are many signs that show that two people, or a person, is in love or is (maybe in secret?) a couple; one is what you pointed out. Changing from formal addressing to a more personal, familiar one can be a sign. That she is addressing him as this when she gets emotional is a sign. She doesn't think it's proper to date someone you work with, right? That would be why she complains on when he calls her by first name. It seems to me that she doesn't mind now and then.
Another sign is that they are starting to care more about each other, more than with ordinary friends. You didn't mention this but keep your eyes open; even as small questions as where he/she is can show that. If the question is in a kind of "by the way" tone then it's probably nothing. However, if it is something else then it can be. Look for the after reaction. If you say that he/she is on her way, look for relief. If you say that you don't know, look for concern.
Lastly; I think that your friends really are feeling something more than friends for each other, but hasn't realized it yet. I personally think that you should only observe how their relationship is evolving but if nothing happens, and they look unhappy… Then you can play matchmaker.
-- Ann
Ps: No, but I'm looking. Central Café, three at Saturday?
A/N: Yeah, Annie is very to the point. And I don't think that I'll be writing anything more of this, but tell me if you think I should change something. I had trouble writing this as my mouth hurted (I got braces today)