I was about to spend hours calculating BPMs for my thousands of Eurotrash songs. Then it occurred to me that somebody must have already written a freeware program that would do this automatically with digital music. I was right. And you can even view the final results in a "beats per minute" column in your regular file view. Fantastic! I am DJ King Pigeon, and I approve this message!
Download it here: http://www.mixmeister.com/bpmanalyzer/bpmanalyzer.asp.
UPDATE: Unfortunately, it seems to be skipping the m4a (i.e. iTunes files). Will see if there is a workaround for that.
UPDATE (10 January 2010): Seems there is no software out there that can calculate iTunes BPMs.