I bought one that had a visibly faulty screen. So I relocked it using bootneuter and restored it to apple's 1.1.4. I returned to the shop and they replaced it. However they took my name and address for the exchange.
Will I be getting a call in a few months asking why I haven't activated?
No. When I bought my iPhone, i didn't enter into any contract to activate it with O2 or not to modify it, or anything. And in the UK, Apple have had to be specific that warranty is NOT dependent on activation - better consumer protection than in the states, it seems.
I bought a piece of consumer electronics that's mine to use as I want, though if I mess it up by modifying its firmware then clearly that IS my look out.