Not at.

'Excited by' and 'excited about' are both fine, but I think 'at the prospect of' trumps them both. So I would vote for 'at,' even if the others are probably not wrong.

You could just say 'very much looking forward to' and save yourself lthe trouble. (-:

*f5* Oops, I didn't see Jurist's post. Maybe I'm too tired to think straight; see what others say.

"at the prospect of" is very good.
"Excited at" is not good, generally.
Not sure whether and how they work together; maybe excited at the prospect of has a meaning that is different from excited about something and excited by something.

Normally I'd say 'excited at', but I see nothing wrong with the others.

We used to talk about this sort of thing in linguistics seminars (it is, after all, a matter of world-shattering importance). The use of 'by' (the preposition of agency in passsive structures) automatically turns the word 'excited' into a past participle rather than an adjective, and the fact that we now have a verbal structure could, it is argued, subtly influence the meaning. I say 'it is argued', but I'm not prepared to argue it.

I'd say:
"excited at the prospect of working with you" or "excited about working with you",
but would not use either "by" or "about" as in the examples given. I don't mean to say they are not correct, just that I'd never use them.

Is "excited" appropriate? I'd write it in a mail/letter addressed to someone I'm on first-name terms with e.g., but not to a large corporation or company. "very much looking forward to" would be better in the latter case IMO.

Yes, actually, that's sort of what I was thinking when I suggested 'looking forward to.' 'Excited' might be a little overeager, a little young-sounding.

And I think escoville's point is probably right that 'excited by' could sound a little more like 'aroused by.' Not a lot more, and I wouldn't argue that everyone would think that either, but there could be that slight tendency.