You need to be careful about WHOM you request the ARP info from.

If the workstation is on a different subnet than the querier, an ARP will cause your router to respond with a "Proxy ARP", and you will end up with the mac address of the router - this is usaually a GOOD thing, since the router will help you send traffic to it's destination, but is that what you really want? The original poster's intent was not clear.

In the Windows world, you can get a remote machine's MAC address running

NBTSTAT -A <IP-addr>
on your workstation.