From about three years ago all operators signed up to share the barring of stolen mobiles. There are various classes of barring available and for some unknown reason not all operators put known stolen ones onto the "black" list. The details of every IMIE are available to all operators and a total blocking could be enforced on any mobile.
Using the net to change the IMIE is not an option as it is physically part of the phones hardware. At least I have never heard of a "visit the net and chabge your IMIE" site. What do you change it to?
Unlocking for multi operator use is generally easy and costs a couple of quid. However, new phones are getting more sophisticated (as are the unlockers), and the days of picking up a new phone and having it unlocked in a couple of days are well gone. An example is the 6630 which although being out for some time now, is not easily, or cheaply unlocked. In fact in most mobile hacking circles, a very high price is paid for the initial unlock software. There are very few people who are known to do the work, and people pay a lot of money initially just to have an unlocked phone from these people.
Unlocking hundreds at a time (bought cheaply on prepay) is a very lucrative business.