I just saw this on NewEgg:
EVGA 112-CK-NF77-A1 LGA 775 NVIDIA nForce e-7150/630i HDMI Micro ATX Intel Motherboard, retail box.
It's gotten good reviews on the MythTV forums so it may be a good choice for a LinuxMCE box.
With mail-in rebate the total is $64.99USD.
Edit: The $20 rebate lasts for the entire month of May.