how do I create a partition for SnowLeopard without wiping Lion?

[Janit]

It would appear that non-destructive repartitioning became available as of Snow Leopard (not sure if it was even earlier).

Check here for how to do what you want to do:

http://reviews.cnet.com/8301-13727_7-200...g-to-lion/

http://www.pcmag.com/article2/0,2817,2390348,00.asp

A similar strategy was recommended to me for adding an XP partition to a Dell Mini I have running Snow Leopard. Cloning a backup is usually recommended before such an operation.