I hope we had all gone through this situation, where IMEI no doesn't do anything good at all!!
Even though its said that it helps in tracing the thief, or even to block, has anyone got their mobile back??? Seems getting the work done with our police itself will consume the cost of mobile with all the bribes involved!
Now with smart phones most of our email accounts and IM services are synced with the device. What to do if you lose your phone, and you havent had enabled the password or pattern lock?? The one who gets your mobile would be able to access all your information, may be even your bank statement (if you have one in your email)
Keeping this in mind, I would advise all smart phone users to install some kind of mobile security and anti theft software like Avast.