Are 6Gb/s SAS drives compatible with 3Gb/s SAS interfaces?
Yup, here is the info I found: http://www.dell.com/downloads/global/products/pvaul/en/6gb-sas-transition.pdf
6Gb/s SAS is designed for backward compatibility with 3Gb/s SAS and 3Gb/s SATA
Here is anandtech testing a 6gb/s drive in both 6gb and 3gb controllers:
http://www.anandtech.com/show/2866/3 (sata, but should be the same)