The OEM license validates the hardware it's on by verifying a number of different parameters such as the hard drive, the Ethernet MAC address, etc. etc. You might be able to swap the hard drive out from under the OS (to upgrade to an SSD, for example), but moving to a new motherboard I think would not be kosher.
If you would like to be able to move Windows to a new machine at some point in the future, then you should buy a retail license rather than an OEM one. Although with the price difference you may find it just as cheap to start with an OEM license now and then buy another OEM license when you decide to move.