I noticed that since switching to Snow Leopard, CrashPlan was using over 500 MB of RAM in some cases.
Matthew Dornquast posted some good info on the CrashPlan support forums related to the memory footprint of CrashPlan on OS X (and on JVM’s in general).
CrashPlan doesn't actually NEED all that ram, it's not using it. Shrink the amount allowed to something smaller.. 80MB, 100MB, etc. How much you need depends on how much you're backing up. I would start at 100MB and work from there. You'll know you've gone too low if you get out of memory errors in the log. For small backup sets, you can do 50MB.
So far it seems to be working fine.