Installing Unity on Debian
Work is taking place to package Unity for Debian, but once that is complete, to run it on squeeze someone would need to back port it. That might not be possible, but if it is, it will certainly be very difficult, as there have been some major transitions in the library chains since squeeze was released (not least GTK3).
You will end up with so many libraries backported you will probably have been better off moving to Debian testing instead.
See also this Debian ITP bug, where one of the people who was working on this spells out how difficult it will be to get it into Debian at all at the moment.