So I just upgraded my site from 2.8.4 to 3.1 (quite a jump, I know). It was actually painless.
I had a question but I just solved it myself -- but I'm posting this here so it shows up in search results for others.
My theme previously included a wp_loginout function as such:
<?php wp_loginout('',''); ?> After I upgraded, nothing showed up there.
Then I asked myself "why am I including those empty parameters anyway?" I changed it to
<?php wp_loginout(); ?> and it works fine again. Hope this extremely minor thing helps someone... :)