I've looked around on the interwebs but I can't find any sort of answer. I just picked up a Mac Mini and I'm going to throw Lion Server on it for my network.
Most of my computers are running either Leopard or Snow Leopard (one iMac is a PowerPC so I can't upgrade to Lion).
I'd like to use Open Directory for all my network authentication but I want to make sure that the Lion Server can authenticate my iMac running Leopard.
I've not tested Lion server directly, but I've done variations of this with just about every other version of Mac OSX client and server and its been fine. I've had Lion clients connecting to an Open Directory domain with a mix of Snow Leopard and Leopard Servers just fine.
Open directory is based on a fairly mature set of principles and standards and - despite gui changes - hasn't changed that much at heart. I'm fairly sure it will be fine.