Like mentioned above, it cannot be used as an internal mounted in a normal sled, but in addition to mounting it in the spare optical bay there is one other option to use one or more of them while keeping them in their heatsinks.
Using an external hard drive enclosure (note that the case needs to be one in which the drive is connected inside the case with cables, not one of the more common "slide into place" SATA enclosures) gives you another option of using it with either:
A PCI-e SATA card or...
A SATA port extender kit (Newer Tech, or the new Sonnet Tech kits like
http://www.sonnettech.com/product/esataextendercable.html ) that make the 2 unused SATA ports on your logic board usable as external connections.
I am using one that way and it works fine, but my experience with it has been disappointing when compared to some of the newer generation standard 3.5 drives (Western Digital 640G, Samsung 750G and 1Tb F1 drives) which have tested right at the same speed as the Velociraptor for most tasks on several review sites.