>>>Since we have TSM and a tape robot, is there a way to define some sort of
virtual filesystem on tape so that I can easily move files from one server
to another through this tape filesystem (since I don't have enough raw disk
to accomplish the moves)?


?? What are you trying to accomplish, exactly?  If you want to move files
from one client machine A to another client machine B, you can already do
that with the TSM client

Backup the files on client machine A

Start the TSM client on machine B this way:

cd into the TSM client directory (program files\tivoli\tsm\baclient on
Windows)
*       dsm -virtualnodename=CLIENTA
*       You will be prompted for the client nodename and password; overwrite
it with a TSM admin id and password
*       TSM will display the file tree for CLIENTA
*       Select the files you want to restore
*       You MUSt specify an output destination, not "restore to original
location"

That is just the most direct way to do it.  You can do the same thing with
the command line client, even set it up to run on a schedule!

>>>Also, has anyone used the TSM APIs to create a HSM for windows?

Yes.
The product was developed by OTG, called DiskXtender.
OTG was bought by Legato
Legato was bought by EMC

It may still be available, but I saw one announcement that implied they are
dropping support for TSM in 2006.  Anybody know for sure about its status?

(BTW, I have looked at HSM for Windows for a couple of my sites, and in
general, disk is cheaper than any HSM solution EXCEPT for sites that are
collecting new data at the rate of so many GB per day so that it is
impossible to buy disk fast enough.)