My understanding of spider behavior is that it doesn't matter if an internal link is relative or absolute - the spider will convert the link to whatever form it uses regardless of the links format on the page. I seem to remember on an old blog from Matt Cutts saying it doesn't matter, but I couldn't find the link so I could be wrong. I have seen the issue raised a few times on Google's boards and the answer has always been that it doesn't make a difference.

However, there is one important caveat. If you are using relative links, you must make sure there are absolutely no canonicalization issues on your site. One of the biggest ways for these issues to be magnified is Google crawling the wrong version of a page, and then indexing a series of relative links that are on the wrong form of the subdomain.

For full disclosure, I generally use FQDNs for my internal links. Personally I find them easier to manage when dealing with dynamic sites and dynamic templates.

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The best way to learn anything, is to question everything.