No such thing as a Frozen ATPL in the US. You may take the written before you meet the applicable aeronautical experience requirements but upon meeting those requirements, you have to do a flight test in order to exercise ATP privileges.
The Frozen ATPL in the UK is named such because (at least in the days before JAA, not sure about now), the only hurdle you had to jump between CPL and ATPL was the written exams. Once you passed the written exams, there was nothing more you had to do other than meet the aeronautical experience requirements and send in the application.
In the US there is an ATP level flight test, the successful passing of which results in the immediate issuance of your ATP certificate.
By the way, the ATP level written test is NOTHING like you'll have to do in the JAA system. There is only one test and all subject matters are covered in that test, and the subject matter does not go anywhere near as deep as the JAA equivalent. There are no training prerequisites to taking the written test, just study as much as you want to, then go take the test.
Any other questions, don't hesitate to ask. I have a UK CPL and US ATP.
Ray