Just a quick message to everyone running Time Machine with the new Steam for Mac client:

Steam creates a large (~4GB) folder in the current user's documents called 'Steam Content'.
This folder seems to be regularly updated (especially during a game download).

/user/Documents/Steam Content
I would suggest adding this folder to the Time Machine Exclusions list as it can quickly eat up the free space on your Time Machine drive.

Hope this helps - I was a little stumped when I came home today to the 'Backup Drive is Full' message!