This is a fairly common MOF edit nowadays, and I thought I had this on my blog already. Anyway, I didn't so now I do 
This MOF edit will inventory the client cache details for your SMS or ConfigMgr clients. It's only a reporting class so that means if you're going to inventory clients assigned to an SMS 2003 site, you don't need to manually compile the SMS_def.mof after adding this information (and you never have to do that for Configuration Manager sites).
//-------------------------------
// Client Cache Reporting Class
//-------------------------------
//The following line is not needed for Configuration Manager hardware inventory modifcations
#pragma namespace (\\\\.\\root\\cimv2\\sms)
[SMS_Report (TRUE),
SMS_Group_Name ("Client Cache"),
Namespace ("root\\\\ccm\\\\softmgmtagent"),
SMS_Class_ID ("MICROSOFT|CLIENT_CACHE|1.0") ]
Class CacheConfig : SMS_Class_Template
{
[SMS_Report (TRUE),key] string ConfigKey;
[SMS_Report (TRUE)] boolean InUse;
[SMS_Report (TRUE)] string Location;
[SMS_Report (TRUE)] uint32 Size;
};
You can get this MOF edit (as a .txt file) by right clicking HERE and selecting Save As...