Add smsc95xx.macaddr=XX:XX:XX:XX:XX:XX to the cmdline.txt file, where the X's are your old MAC address.
This worked for me, but wasn't as straight forward as I expected.
First, I'm not a Linux person (and don't particularly want to be either).
Trying to edit cmdline.txt on the live system failed due to write-protect.
Trying to edit cmdline.txt with notepad on a win7 pc (initially) failed because regardless of how I did the edit, I still ended up with a cr or lf and the "smsc~" command on the next line.
I remembered a similar problem years ago (SCSI drivers I think) and the edit that worked was to insert the new command into the start of the file, not the end. For some reason windoz doesn't insert the lf (and stuff things up).