Thanks, fmalcolmr--we don't always know what a member's intent is and just want to err on the side of caution when this subject is brought-up (it raises the radar for a lot of folks), so please know that its not simply directed at you, per se.
I think the issue would boil down to if one is modifying one's device (in whatever manner) to use the services of another company without their consent / notification and/or proper compensation.
I'm not an expert by any means, but a quick search yielded a good thread over on XDA (especially post #7):
esn change legality? - xda-developers
I think the rules are a bit more lax over in Europe, but over here in the states, there's red tape everywhere
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Cheers!