There is something to that - due in the next Windows Server 2008 R2 and Windows 7 is an update to Offline Files and network copying, where the server will keep hashes of every file, in smallish chunks.

The idea is that if you copy two similar files from the server, it can check the hashes and tell your client to use parts of a file it already has instead of transferring them (or only copy changed parts of a big file).

I have no source handy to find more details about exactly what scenarios it will be used at the moment though.