Wouldn't it make more sense to just include the RAM used for disk-caching when calculating free memory? After all, the whole reason it's being used for disk-caching in the first place is that it's free! The Linuxoids that know what this even means can always explicitly look for the line that says how much memory is being used for caching...
Exactly the kind of thing users not familiar with the way Linux handles memory DON'T consider when they check how much free memory they have. If you're the sort of person concerned with the overhead of zeroing pages then you can look at more detailed memory-usage info to find the details you're looking for.
The Windows task manager has a separate field called "Available". Right now I have 8190 MB total, 3627 cached, 4151 Available, and 537 free (not sure if that adds up perfectly, it was changing while I typed it).
OSX's activity monitor does something similar, it has 4 statuses: Wired, Active, Inactive and Free.
Wired and Active are basically the RAM being used (Wired is a special activity status for stuff which can not be moved to disk), Inactive is the cache (generally programs which were used recently) and Free is… well, free.
When checking used/free, Inactive and Free count towards free, Wired and Active towards used (though that's not always consistent).
