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I'm running ubuntu server 10.04 If i change the mac address by adding (in /etc/network/interfaces)

hwaddress ether 12:34:56:78:9A:BC

How reliable is this? Can someone still determine my real mac address?

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You'll have to define "someone". If anyone is close enough to your PC to get your MAC address (1 hop), they can probably get it if they really wanted. MAC spoofing is just a software feature. The real MAC is usually stored in read-only memory on the card itself. If the machine is booted into another OS that doesn't have the address spoofed, your real address will be shown.

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I know is just a software feature. There is no other OS. The device is sitting behind a personal router. Could anyone outside of my router still determine the real mac address? – Matt Phillips Nov 1 '10 at 23:27
I think my point is unless someone is actively trying to find my real mac address will it show up during normal tcp usage when i define a different one via the interfaces file? – Matt Phillips Nov 1 '10 at 23:31
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@con - "Could anyone outside of my router still determine the real mac address?" No, MAC addresses are not transmitted across the internet in the same way your IP address is. Anyone receiving data will see the MAC address of the last hop (usually their own router) as the source MAC address. MAC addresses are not needed until the packet reaches your LAN, in which case good old ARP ( en.wikipedia.org/wiki/Address_Resolution_Protocol ) is used. – John T Nov 1 '10 at 23:36
@John T. Ok that makes sense. But if someone makes an arp request to my router will that allow them to get the real mac address somehow. I know its theoretical but I want to know how realistic the possibility is. – Matt Phillips Nov 1 '10 at 23:54
@controlfreak123, no. – John T Nov 2 '10 at 0:23
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If you want to change the MAC address at the kernel level, you can use a program called macchanger. It can also randomize the MAC address of a given interface.

Only problem is, if your network depends on DHCP, you'll get a different IP address every time. This can max out the DHCP server's lease cache very quickly.

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