it can be used in snow leopard, however, you must have a certain framework installed (that is used by Lion, and not by snow leopard)
you can try a program called Pacifist - to open the messages.pkg file and browse yourself, the stop-errors are called by the "distribution" file in resources which runs a script to check for version of OS and version of OS on HD
there are only those 2 checks to determine if it can be installed.... and sadly, it cannot without the Lion frameworks (i stopped there as i was bored enough already with it) lol
you ARE in fact able to install iWeb and idvd and front row onto Lion by editing those checks and i have done so and they work flawlessly
messages beta, well, it's in beta.... so check back after a proper release is finalized!
happy hunting/editing!
PS - do NOT try to install messages beta via Pacifist - it will screw up your ichat for good as apple has no dmg for ichat for snow leopard !!!