By default WSUS does not clean up anything in an automated manner. This is not normally too much of an issue for a system with plenty of drive space and a system that does not do definitions. However with FCS thrown into the environment WSUS will sync new definitions three times a day at a size of about 30megs 112Mb each. These definitions are then declined on a normal basis as new revisions come out which means that over a month you could build up about 1.8 gb 9.4Gbworth of old definitions ( 5 business days 3 times a day 4 weeks a month 112Mb 30mb per release).
There is a powershell script out that will allow you to automate this on your systems that you can find at http://www.pulsarit.net/cs/blogs/claudiog/archive/2007/05/16/eseguire-wsus-3-0-cleanup-manager-via-powershell.aspx however not everyone wants to install powershell on their systems. So in lieu of the script there's an .exe now on codeplex that does the same thing.
You can find it at http://www.codeplex.com/WSUS/Release/ProjectReleases.aspx?ReleaseId=17612. It's a small .exe just throw it in a folder somewhere and create a task scheduler task to run this on a periodic basis. This should help you keep the disk space bloat down on your WSUS servers that are serving up FCS definitions.
Revised on 3/5/09 based on corrected numbers for definition sizes from http://blogs.technet.com/kfalde/archive/2009/03/05/wsus-fcs-definitions.aspx So basically it’s even more data then previously and you definitely should use this script to keep your WSUS servers trimmed.