The final version of SL will be available for sale in September. Current builds of the SL beta have been refreshed about weekly lately, and they are now up to Build 411.
Yes, you can install it on another partition for testing purposes. DO NOT install over your 10.5.7 setup, unless you like re-installing from Time Machine backups later when the GM version is released. Apple will not allow SL GM to be installed over SL Beta. You'll have to do a format - install - TM restore - it's probably best that way anyway, considering the wholesale changes in the codebase.
And yes, I am a member of the developer program, but I'm happy to satisfy your curiosity.
JP