This is a bit out of date, but talks about the Search laws and the fourth
amendment in pretty straight terms with lots of references to precedent.

Basically, an American cop can give you a breathalizer, consent or no.
(DON'T DRIVE DRUNK!) but they can't search your car without your consent.
However, if you give them cause, they can deem you impaired and can search
without your express consent.   At that point it's up to a judge to deem
whether the search was constitutional.

http://www.mdle.com/WrittenWord/rfinley/finley15.htm

-eric