Before I am assaulted for asking this question, the reason it is of interest of me is not because I have a stolen phone, but T-Mobile US is now blocking certain data plans by identifying handsets by IMEI number.
I recently bought a unlocked HTC Magic and inserted my SIM card which worked just fine on a T-Mobile Shadow (HTC Juno). The data plan worked for a few hours, and then was blocked because the T-Mobile network recognizes these phones IMEI as a G1. The data plan I actually had (per the recommendation of a T-Mobile rep), was a Blackberry plan with an International E-Mail Roaming package (as I travel frequently internationally). After many hours on the phone with them trying to find a solution, T-Mobile determined there was not a solution and I could not use the HTC Magic with my plans.
Given these changes on the T-Mobile US network are relatively new, and the imminent growth of Android handsets, I believe there could be a market for such an application.
Lastly, according to my research, in the United States, there are no laws that exist which bar changing of an IMEI number. So, to all of you vigilantes out there, please leave law enforcement up to the police.