yes it works fine (so far) We each bought one this week and had no problem registering them to our amazon.co.uk accounts.
Whilst amazon might block things in the future they'd have to be nutty to do so. after all you're registering it to buy stuff in the uk from the uk site. should they really care whether you are reading it on your pc, ipad via their apps or a US bought kindle? so long as you're making them money
We've already bought several books since getting home, if amazon did block it in the future i'd just start buying from non-amazon stores and converting to mobi for use on it so it's not really in their interests to do so (not that companies always do the logical thing
You can't take advantage of the special offers of course, but you are still been advertised to and they're gaining brand awareness such as the dove ads, so i don't see even the kso as been an issue. Hopefully in the future they'll release a uk kso and allow the ad server to be switched so we start seeing ads we can take advantage of though.
If amazon didn't want to allow usage of US kindles in the UK i'd expect their site to mention that US kindles can only be used with the .com site. Much like how DVDs have to state if they're region locked which regions it can be used in.