From wikipedia's MAC address :
A Media Access Control address (MAC
address) is a unique identifier
assigned to network interfaces for
communications on the physical network
segment. Logically, MAC addresses are
used in the Media Access Control
protocol sub-layer of the OSI
MAC addresses are most often assigned
by the manufacturer of a network
interface card (NIC) and are stored in
its hardware, the card's read-only
memory, or some other firmware
mechanism. If assigned by the
manufacturer, a MAC address usually
encodes the manufacturer's registered
identification number. It may also be
known as an Ethernet hardware address
(EHA), hardware address, adapter
address, or physical address.
Although intended to be a permanent
and globally unique identification, it
is possible to change the MAC address
on most of today's hardware, an action
often referred to as MAC spoofing.
Unlike IP address spoofing, where a
sender spoofing their address in a
request tricks the other party into
sending the response elsewhere, in MAC
address spoofing, the response is
received by the spoofing party.
However, MAC address spoofing is
limited to the local broadcast domain.
In the IPv4 world, the MAC address of your computers is not propagated or detectable beyond your local network. This means that under normal browser operation (without plugin intervention), the MAC address of any computer positioned behind the router is not sent beyond the router, and so cannot be used to track you over the Internet. However, the router uses it internally in order to identify the local computers.
In IPv6, the 64 bit "host" part of the full 128 bit address is often automatically generated from the MAC address, and hence often is visible to the server one connects to. This means that outsiders may trace the MAC address of your router and your Internet-connected computers. IPv6 gives a public, routable address to every last PC, server, printer or network device (unless the router does NAT processing, which is rare in IPv6).
If Private IPv6 addresses (defined in RFC 4941) is turned on, the
exposed identifier is generated as a random number rather than from the MAC. This is the default on many operating systems, but can be turned on and off. For further info see
How to avoid exposing my MAC address when using IPv6.
In any case, your ISP always knows who you are and can trace anything you do, unless the traffic is encrypted by using a VPN.