Nope, I have books on my Kindle that I've bought from both locations. 
The one thing I don't know is about Amazon's TOS. I don't know if, as a permanent resident of the UK, I'm violating anything by switching accounts/locales to buy things in the US. It allows me to do it, so I'm guessing it's not an issue, but I haven't actually read through the entire legalese.
I believe you are. I got a strongly worded email from Amazon whilst using a US kindle with US account in the UK and they banned me from shopping at the US kindle store whilst I was in the UK as a TOS breach (It wouldn't let me purchase anything at all). I got the boy to call them up and tell them I was an American Citizen travelling in the UK and they unblocked it, but still...annoying

If I'd been permanetly resident in the UK (I am but whatever) they wouldn't have unblocked it.
Right now I only have a US kindle Account, books seem to be cheaper on there anyway and I've not come across anything I want that isn't on the US store. You might get away with it a bit more if you're using wifi, I use 3G almost exclusively. Dunno, that's just speculation.
Also my US amazon account has my UK debit card attached, and my UK billing address, by a US shipping address attached. Not that that matters as Kindle books are obviously delivered electronically