OS X :: Spoofing/changing MAC Address?

I'm getting a new iMac delivered on Tuesday, but I'm in student accommodation, and the internet (which costs �90 a pop!) is set to a certain mac address, namely the one for my old macbook.

I know you can change a mac address (I think, based on some searching that it's with the
Code:

sudo ifconfig en0 ether xx:xx:xx:xx:xx:xx command), but does that change it change it permanently or just make it look different? Say, for example, if I had to reformat, or say, boot into windows with boot camp, would I need to change it again or would it be set permanently?