This have been touched upon before, but i have now actually tested it.

I installed wow in a folder called wow1.
I then copied that folder, renamed the copy to wow2.
I then deleted the content of the data folder of wow2 and created an
NTFS junction point in it's place, pointing to the data directory of wow1.

I now have five copies of wow taking up very little more space then one
copy of wow. I also only have to patch one copy, since they are all really
the same. Because i'm a bit lazy the four clones share the same AddOn
directory.

I'm not sure how this behaves performance wise compared to having five
complete copies installed, but the five copies (sharing the data directory)
ran very well on my Pentium M 1.7 GHz with 2 GB of RAM.

I got about 30-40 fps on my main and 10-12 in the clone windows.

I expected more problems from this, especially from sharing the AddOn
folder among the clones. But i haven't seen any problems so far.
I'll come back when i do :-)

http://en.wikipedia.org/wiki/NTFS_junction_point