Also, I don't think this is a problem related to the network card in use. I can ping the other computers just fine.
If the network shares worked for you in 10.5.1 or any other Leo version, I'd try to copy the filesystem folder into this install. especially the URLMount folder inside it..

I'm gonna try this when I get home, that any maybe copy thes ones from 10.4.8 if it doesn't work.

Is everyone elses shares working?

I'm running a DFI Infinity RS482.
North bridge: ATI Radeon Xpress 200 RS482
South bridge: ATI SB450
Onboard network: Realtek RTL8110SB Gigabit PCI LAN (works native with 10.5.2)

What's your setup Mikeintosh?