We use a StarWind SAN which has the concept of thin-provisioned disks that grow as needed. You can allocate a 4TB drive but it starts off tiny and grows as blocks are written to the virtual disk (via iSCSI).
The virtual disk used for our main file system has grown to 1.5TB and has plenty of virtual space left (2.5TB) but disk space on the SAN is another matter - it's getting a bit tight. That's the downside of thin provisioning - you can overcommit disk space.
So we're busy archiving old folders off the main disk system to the archive area.
However, this will only make any difference if Windows 2008 re-uses the deleted blocks before it adds new blocks to the disk when new files are added.
Is this the case or are we wasting time archiving (aside from the benefit of keeping things tidy) and need to consider expanding the SAN disk soon?