Windows 2003 DHCP/Linux (dhcpcd) client

Here’s what’s been racking my brain for the last week–hopefully someone can help.
I have a WIN2K3 DHCP server dolling out IPs which works except for one machine that I want to use a reserved address. Capturing the packets looks right, the MAC matches what I have in the reservation, but the WIN box offers another address from the pool, which the client then accepts. Other reservations are working correctly. The only clue I can find is in the Windows DHCP log which shows a 36 character(!) MAC address from this client, with the last 12 being the actual MAC address. If I had to guess, I would say it’s the server misinterpreting, not the client sending wrong. Appreciate any ideas!?!?