Open an encrypted HDD/USB on Leopard? Is it possible to access an external HDD or USB drive encrypted on Lion, through a Mac running Leopard?

In other words, is FileVault 2 encryption on volumes/partitions backwards compatible? (It really should be.)
No - OS X older than Lion cannot read or comprehend/mount/use a Core Storage volume such as used with FileVault 2.

Someone could reverse engineer the protocol and write drivers for older OS or different OS, but I haven't even seen a start towards that effort in any open source project.