Yup SATA and SATA II are fully backward compatible, both ways. You just wont have SATA's II benefit if you plug a sata drive in it!
<A HREF="http://www.anandtech.com/storage/showdoc.aspx?i=2450" target="_new">Here's a perfect read!</A>
Asus P4P800DX, P4C 2.6ghz@3.25ghz, 2X512 OCZ PC4000 3-4-4-8, MSI 6800Ultra stock, 2X30gig Raid0