I am flying Friday and feel totally confused on the procedure. Did read some mention that you should enter the UK passport in the airlines online check-in info.

It confuses me as I have given the airline the US passport info which I assume gets past along, is this the API. Also some people say use the UK passport internally. The law is the enter/leave on the US passport. Don't want to confuse the UK border control as don't they now record the API.

Has anyone done this recently with a US/UK passports?

In 2009, I didn't use the UK passport as I flew into Edinburgh and noticed the lines were a lot shorter. However, the border officer noticed that I was born in the UK and asked why I didn't use the UK passport. I told her it was due to me getting worried about what happens when traveling back to the US when they don't see the stamp etc.

Don't confuse the US...
Enter your US PP info. Use your US PP at all times other than entering the UK, when you use your UK PP. Only present your UK PP on leaving the UK if you're asked for it, otherwise, just use your US PP.

I do this all the time, it's a non-issue.

This is from the State Department:
http://travel.state.gov/travel/cis_p.../cis_1753.html

"Most U.S. citizens, including dual nationals, must use a U.S. passport to enter and leave the United States. Dual nationals may also be required by the foreign country to use its passport to enter and leave that country."

I know the following works, we did it about three weeks ago.

Give airline your US passport info only.

Use US passport for check in on flight to London.
Use UK passport to go through UK Customs and Immigration.

Use US passport to check in on flight back to US.
Use US passport to go through US Customs and Immigration.

Thanks. I will the suggestions above a try.

Just returned back from my family reunion in the UK and wanted to share my experiences.

I took the advice from the people who replied and had no issues. Below is what I did.

1. Left the USA with the US Passport, giving it to the airline.
2. Entered the UK on the UK passport. In Heathrow Terminal five, I noticed that UK Border Control only had one line for both non-eu and eu people. I asked the border officer that I had two passports and he said it was up to me which one to use. He did warn that sometimes the US officers will look for the stamp in the US passport on US return and can question why it was not there. Decided to use the UK passport.
3. On my return, showed the UK domestic ticket desk and UK overseas flight desk the US passport.
4. In USA, shows the US passport to immigration and had no issues and no questions about why the passport was not stamped.

It worked great.

Thanks for all your help.

He was wrong about that - at least I have never heard of it happening - and even if it did happen - so what - you are a US citizen returning to the US with a US passport - they can't refuse to let you enter the country and, anyway, the US can't control whether or not a foreign country puts a stamp in your US passport or not, and they really don't care where you have been as long as it wasn't Cuba ...

Exactly. None of their effing business whether a non-US passport was or wasn't stamped as long as you're showing them a US one.