Linking together unrelated sites will help your ranking a bit, but obviously not as much as getting incoming links from related sites. It is definately worth doing, but is not a substitute for genuine reciprocal linking.
Sprinkling cross-links throughout your pages is usually much more effective than taking one page on each site, and packing it full of links. That way, it won't look like you're running a linkfarm.
-- Jeremy Maddock SEOMix.com - Search Engine Optimization Tips My Blog - Business, tech, and politics from a webmaster's perspective
It's very helpful to 'seed' PageRank to a new site with links from within your network, that will help you be able to make link exchanges in the future. (I don't necessarily agree with basing link exchanges on PageRank, but it's a fact of life) It's a good general rule of thumb to keep your in-network links to a smaller number than your out of network links, speaking of both inbound and outbound links there.