As wise monks have already stated above:

• arp only works for IP address from your local (segment|subnet|broadcast domain)
• if you try to arp-resolve an IP address from a different subnet, the resulting MAC address will be that of the router interface nearest you.
• MAC addresses are an Ethernet (not PPP) thang

Token-Ring and FDDI devices also have MAC addresses but, IIRC, you get into (big|little)-endian stuff there.   Arcnet and Appletalk used their own funkomatic hardware addressing schemes.

If you are indeed looking for the MAC address of a local-to-your-LAN-segment device, (code) MAC n' IP cheese includes code of OWTDI.
    cheers,
    Don
    bumbling toward Perl Adept
    (it's pronounced "why-bick")

Update: I was thinking of IPv4 only.   As lhoward points out, IPv6 is substantially different.

Update 2: Working from riffraff's Re: Re: Re: How can I find a MAC address from aremote IP ? post above, I found that (code) MAC n' IP cheese can work for remote segments just by replacing the "Querying ARP table..." section with snmpwalk <router_name_or_address> <community_string> IpnetToMediaPhysAddress

Woo-hoo!   8^)