Stefan Gossner has published a great article on the subject:

In an earlier article I have discussed that all SPSite and SPWeb (and potentially also PublishingWeb) objects need to be properly disposed to avoid memory pressure situations. Roger Lamb has a nice summary of coding patterns which will actually cause such leaking objects and also provides information about how such code would have to be adjusted to avoid such leaks.

In this article I will provide some details about how a sharepoint administrator is able to isolate if the site contains custom code which does not properly dispose these objects and also how to isolate the components that are leaking these objects.

Go to Stefan Gossner's blog to read the full version - http://blogs.technet.com/stefan_gossner/archive/2008/05/07/troubleshooting-spsite-spweb-leaks-in-wss-v3-and-moss-2007.aspx