Yes, Lion supports all Mac Pros, no matter how old.
The minimum requirement is a Core 2 Duo processor (because that is the first Intel consumer CPU to support 64 bits processing). Because even the oldest Mac Pro has a more advanced processor (a Xeon to be exact), all Mac Pros ever sold are supported by Lion.
Actually the first intel consumer CPU to support 64bit was the Prescott revision of the Pentium IV in 2004. This was followed by the Pentium D and Pentium Extreme Edition as well then in 2006 the Core 2 came. The Core duo and solo were nothing more than rebranded Pentium M cores which themselves were a hybrid of the bus architecture from pentium 4 and a heavily revised pentium III architecture.