OK, I'll tell you what I tell the folks who work for me: the clock starts running at the end of the conference. Sending an email now only to see this contact in the bar 2 hours later can be a little awkward.
What I do, especially if the contact is a potential client (as opposed to vendor) is send the email (or fax or phone call) with at least one good idea about whatever problem it is that you can solve for him. Remember that there is almost certainly someone else who can do whatever it is that you can do, so differientiate yourself somehow. With me, I can sell service all day long, and there aren't many people in my industry that can keep our pace with our hit ratio. Most of it is smoke and mirrors, but my clients don't really care.
One other thing, especially if this is a client - there is absolutely no shame in saying "I don't know, but I'll get an answer for you quickly" so long as you deliver on the promise. In business, never ever ever just make up an answer or be embarassed to admit that you don't know something. Admitting you don't know and finding out quickly for a client is a sign of strength, not weakness. Speed, however, can play a role.
That's my free advice. It may or may not be what you've paid for it.
