My wife and I moved to PA from the UK in August and had similar questions to those of the OP, at the time of the move.
The original post speaks to the situation in IL, and some of the things we discovered, in PA, are different. I offer our experience to those looking for PA info, but with the caveat that DMV state rules will change occasionally and the first port of call should be the State's DMV website!
In PA as of now (Dec 2009) the UK license is valid on its own for 60 days after arrival, and 1 year in combination with an International Driving Permit. We didn't bother applying for an IDP... mistake number 1. We should have applied for an IDP before we left. It would have given us more time to take the test and get a PA license...
Mistake 2 was not renewing my UK license before leaving our house in the UK. It has now expired and I no longer have a UK address to use for renewal...
After the relevant period (60 days or 1 year) a UK license holder in PA is required to apply for a learner's permit (involving form filling, evidence of residency etc and a multiple choice knowledge test based on the State's drivers manual, available off the DMV website and worth reading). I used an iPhone app called Driver's Ed, which gives sample questions and is available for all States. It got me used to the terminology and style of questions, so we understood the questions when it came to exam time. There may be other software that does the same thing and it definitely worked for me and wife.
Once in receipt of the learner's permit, you can apply for a practical test. This is pretty straightforward and involves driving safely, lots of mirror work and parallel parking.
The experience we had was not great. The officials at the DMV we went to were not used to foreign license questions. At one stage we were told by one official we could swap out our UK licenses for PA licenses straight away. In the next room we were given the facts. One must understand that people working in DMVs are not in the most fulfilling careers and tend to show this....
I believe that there is no state in the US which will swap out a UK license without taking a written and practical test.
If however you have a French or German license you are in luck in PA at least. They can be swapped directly.
Good luck to the OP and be ready to have an experience which will take a few years to be able to look back on and chuckle over...