Tracking the MAC Address

If we know the MAC address of a particular non-windows system which is in a different network segment (e.g. separated by router(s)...), is it possible to locate the system and find out its IP address? If so how and what tools could we use?
On the other hand, how do we get the mac address of a system in a different subnet (separated by routers...), if we know its ip address?
Thanks

I do not know of a direct method of obtaining the results for either of your questions, however if you have access to the DHCP server from which either machine obtained their IP address (if static addresses not used), you will usually be able to get an IP-MAC address mapping from the DHCP lease database.

If you do happen to have access to the switch to which the device is connected to and it is a managed switch, you may be able to access the IP-MAC address mappings. Chris has shown how to do it for a CISCO 2950 Catalyst in this post

From my experience (i could be wrong), if the network is separated by routers then when using reverse arp or arp you will get the IP and MAC address of the router nearest to you. The best way to find out that users ip from the mac address and vice versa u would have to have access to the router that is in the same subnet as the device you arelooking for.
Please correct me if i'm wrong.
Hope i helped,
Cheers

Which is why I have not recommended ARP, but instead the use of the DHCP lease database or access to the switch to which the machine is connected to.

What about using something like ethereal, or some other sniffer, would that work?

yes, ethereal should be a good tool to basically find out everything about the network, specially IP addresses and their respective MAC addresses. keep the two end devices online, ping one from the other, and ethereal would easily tell you the respective MAC addresses