Ever since you released the most recent patch (6/29/2010) the game will verify it, then say the steam server system is busy and to try again. and i have already contacted steam about this problem and they told me to come to you to ask.
system:
Macbook 2.4 GHZ intel core 2 duo with 4 gb ram and osX version 10.6.4
system:
Macbook 2.4 GHZ intel core 2 duo with 4 gb ram and osX version 10.6.4