I just printed out my DS-82 form. This is the form I need to renew my passport. It's also the form which will lead to me being legally required to carry two passports when I leave the country. That's because my UK residence permit will be in my old, expired passport, not my new one. I'll need to present my new one to prove who I am and the old one to prove I can live here.

In a year and a half, I can apply for my permanent residence and work permits and those will go in my new passport, thus allowing me to only carry one passport again. However, a year after that, I'll be applying for UK citizenship and, when granted, I'll have to carry both US and UK passports for when I visit the US. The US requires US citizens to use US passports in the US, regardless of their other citizenships, but I'll have to carry my UK passport for when I return home.

This raises an interesting question, though. As a US citizen, it's illegal for me to travel to Cuba without State Department approval (Iran and North Korea are no problem. Go figure). As a UK citizen, it's quite legal. If I only have a US passport, Cuba stamps a separate page to slide into my passport which I can remove when I go back to the US, but otherwise, it's still illegal. So presumably I'd want to avoid those legal shenanigans and hand the Cuban officials my UK passport, but what happens if I visit the US later? If they see my British passport, they would presumably want to thumb through it. Could I be charged with a crime for something that it would be legal for me to do as a UK citizen, but not a US one? Presumably I should beg the Cuban officials to just give me the paper visa to slip into my passport rather than stamping the UK one.

However, this minor annoyance also means interesting possibilities. For example, if I visit Israel, other middle east countries might bar me for having an Israeli visa in my passport, but I can just switch passports!

Two and a half years down and two and a half years to go.
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Cuba won't stamp your passport regardless of country of origin, they issue a paper visa which they stamp instead. You have to actually request them to stamp your passport, and even then they're reluctant to do so.

And as far as I know, the embargo is against spending US-made currency in Cuba, so presumably having worked in the UK for this long, you'd be ok. Not 100% sure on that one though. Either way, I met a number of Americans in Cuba who didn't seem to care much.
I didn't know the bit about Cuba not wanting to stamp anyone's passport, but I'm curious about the "spending US-made currency". The State Department Web site is full of legalese, but in parsing it, it sounds like even the buying of a ticket is illegal (along with spending money), thus making the travel pretty much illegal. Still, it sounds like you're right and this National Geographic article asserts that travel per se is not illegal.
You might also want to pay keen attention to the Obama administration. I believe they may be a bit more open to softening on Cuba than ol Georgie. Either way, they have to catch you first (the US that is) and I doubt that will happen as claudiag is correct. No stamps no worries. Besides as a citizen of the UK eventually it won't matter.

Til then, just go and enjoy the Cuban cigars and rum.
Oh I am sure...

In either case, no real need I think to have any infiltraion/exfiltration plans to appease the current administration.
Being a citizen of the UK is irrelevant if you are a US citizen and something happens to you in the US.

Edited at 2009-01-03 07:17 am (UTC)
Well, that is an interesting question... I'd probably present them my US passport and then chuck the paper later.

Interesting note, the department of homeland non-security now requires foreign travelers, from countries which we do NOT require visas for entry, to fill out a form prior to travel and turn it in... so that we can refuse them entry before they've even boarded a plane. Good times. Yep.
I'll get my UK citizenship this year and have had it in my mind that I'm celebrating by going to Cuba. :-)
You won't strictly need two passports
Rena's a UK citizen now, but doesn't have a UK passport yet (she's only been a citizen for a month.) She re-entered the UK with her ILR visa in her US passport this morning with no problems what so ever.
Having been a permanent US resident on a green card and hold both Irish and British Citizenship (I was born in Northern Ireland) and now having a 3rd passport to renew after taking US Citzizenship, I can only advise this. I frequently travel outside the US to Ireland/UK and in eight years of doing so have never been questioned about not having an entry stamp on my US Passport and have never been made show my 1st/2nd nationality passport. However, I do know that on a green card, it is illegal for a non US citizen to travel to Cuba much in the same way that a US citizen cant. Just food for thought.