It's been recommended to me that you put your business card or some other form of "This bike belongs to..." documentation IN your seat tube. Thieves who know what they are doing would have no qualms about rubbing out serial numbers or anything in the paint. They won't necessarily think to pull the seat off and look in there. I've heard of a few bikes being recovered through bike shops that were aware that the bike was stolen and that the owners name was there.

The scary development in my DH's bike story is that the guy who was selling it on eBay suspected it might be stolen and called the police to check the serial number. The person who took the call didn't even bother to check the stolen bike reports and told the poor man it wasn't reported.