Okay. Hacker attempts with invalid keys are outnumbering real keys by a factor of 2:1 or so. Obviously this is somewhat validating the use of CD keys.
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The actual algorithm is a 64 bit DES key encryption method open to a very small window. The chances of coming up with a real valid key is in the range of billions and billions to one. A random pass through the available keys shows no sign of repeating sequences, it looks like good old chaos (in fact, statisical math proves this).

6:40 update: Graeme added this important bit of information which should make a few of you go "ooooooh" or something:

Secondly, CD keys are "leased", so if you log on through a dialup, play, then disconnect your modem and log right back on, your CD key will have to timeout on your previous IP before you can play again. This timeout period is currently set at 5 minutes.