I would agree with Scottie. As I've said elsewhere we put up a link on our company web site to every site we host. It is in the hosting section and under the general heading of "sites we host". I have always been amazed at how fast Google picks up these links and how they seem to value them.
Sorry azs- we tend to wander a bit off topic around here.
What your sites have in common is the designer, right? That is relevant.
With this in mind we decided just a few days ago to add a list to our SEO service page that says "web site we've optimized". We only did this within the last week or ten days and I have already seem Google report these links.
Every page in our company web site has a PR4 or better. I don't know if this makes the links more valuable, but I know that Google has no problem at all with these links.
BTW the fact that they are from our own web site to sites we host, means that all the sites are on one server. Now we have a couple of blocks of IP addresses, but nevertheless many of these site share a C Class block and the links still get picked up and reported.