I have never personally worked on a project for radio broadcast but I'm sure the standard output file would be a stereo interleaved wav file. However, you should pay particular attention to the level norms, which can vary somewhat from country to country. Your bounce would need to conform with the level norms (peaks and average levels) so I would make sure that you have clarification on this BEFORE you start your mix. I made the mistake once of mixing a project before realising that the broadcast norms were quite different from what I had used for my mix, and ended up having to re-mix the whole thing from scratch.
This article should be pretty useful in getting your head around this if you haven't dealt with mixing for TV or radio before. If the client has no idea about the spec requirements, then you should diplomatically insist on finding who you can speak to in order to have the relevant info.
Please note: When commenting on someone's question or answer, you will not receive notification on follow up comments unless you are mentioned with @Username. So for Andrew Spitz, you only need to use @Andrew
If you're concerned, definitely speak to someone at the radio station where your spot is to be aired. Maybe your client can refer you to whoever he's dealing with there. I did one a little while ago, and i mixed it to -3dB peak with a -20dB average in Phase Scope (set to BBC with 10 sec window). That seemed to work just fine.
Just a stereo mix should be fine too.
I did a bit of googling to see if there were any generic specs for radio spots, and the general consensus seems to be that you can drive by the radio station and shout the spot out the window, and they'll still find a way to put it on the air. It's money for them, right? Why should they make it difficult?