Although the last few posts are quite funny, I'm gonna veer back to the conformist discussion.
I think calling the next day can be bad, and following a rule is always stupid. I recently had an experience that I think shows both. We met at a bar, and she was already a bit drunk. She thought she was rambling a lot and told me several times that she was usually shy, calm, and not all the bad things she thought she was doing. She offered me her number and reiterated the previous comments. Getting drunk, meeting somebody and talking a lot, then receiving a call the next day seems a little extra embarrassing to me, so I waited. If that reasoning is stupid I'm sure I'll be informed
On the second day I was unexpectedly busy and didn't have time to call until late, which wasn't a great idea because she works early.. So I called on the afternoon of the third day, and left a message explaining and apologizing. She didn't call back, quite possibly thinking I was following the three day rule.
Also.. I like MSD's rule.