I've gone back and forth on this one myself.
For the most part, I agree with J-man that it's a good interest level test.
There's someone I used to work with that I enounter at the mini-mart across the street. I want to go out with her, but I don't want to leave it to chance - often months go by before I run into her.
So I called her at work about two weeks ago and she responded that she had to get some family stuff out of the way before going out anywhere in the evening. She suggested I call back, so now I think it's time and if she's wishy-washy again, I'll know what the deal is. If I get voicemail, I'll leave a clear message and if she calls back, great - if not, then next.
I also want to date this really attractive woman at a client site - she's a new ad sales rep. She seemed friendly enough, good eye contact, etc. Intending to ask her out, I called around 3:00 or so Fri afternoon. She wasn't there, so I left a message.
Once again, if she calls back - great. If not, I won't ask again and if I see her, I'll let her bring up the message, if she chooses to.
I figure in either case, if they're sufficiently intrigued, they'll call back.
I have two friends who *instantly* pick up their phones, and I'm sure it's because they're afraid of missing a callback from someone they're interested in.
It's actually a good filter - a gal who really is interested will leave a message if you don't get to the phone in time.
In fact, always being available is an IL killer.