http://www.psiquilt.com/2010/01/sigg...-tutorial.html
Here is another block suggestion. Many of the folks on the board are doing this signature block as an exchange right now. The general consensus is that it goes together quickly and easily. Alternating the direction of the white part can really create an interesting layout. If you scan the Siggy Block Swap thread, you will see a couple of links to layout suggestions.
Besure to use a good fabric pen for the signatures and heat set them. Also, keep in mind even the smartest people refuse to follow directions on signing blocks. You may have to stand over them to make sure they sign the right direction!