I just upgraded this site from WordPress 1.5.2 to WordPress 2.0 – it was a very simple process and it looks like they’ve done a great job in keeping backward compatibility.  All my plugins and my custom theme worked (seemingly) perfectly without any modifications.  The whole process took about 10 minutes (including uploading the new files).  The site shouldn’t look different at all, but the admin is a lot nicer – you can check out the new features if you’re interested…
Let me know if anything seems to not quite work.