Any side-effects when sharing a Time Machine backup disk between Lion and Snow Leopard?

In general, there are no issues or side-effects of using a drive connected in turn to any number of macs with mixes of OS levels since each Mac only writes to its own backup folder.

The obvious exception is Lion style FileVault encryption where Snow Leopard and older macs will be prevented from reading or writing the drive.

The only other problem is if you mix lion and snow leopard on the same Mac and (or) let lion "inherit" the backup of another machine.

In that one case, the contents of the backup are mixed and you can't be guaranteed a restore of system files will work. Your data and pictures and other files will be fine but anything tied to database files or a specific version of the mail app (for example) can depend on versioning.

This isn't really a side effect of Time Machnie changing how it works, but of the OS and app changes.