Some might consider this obvious, but it was hard for me to find. To see your account information in iTunes4, including a list of the music you have purchased, click the button (that almost doesn't even look like a button) with your e-mail address on it on the right, under the search widget (when in the Music Store, not your general Library). If this gets rejected based on obviousness, I'll get over it. But I felt I should save somebody else the time I spent!
[robg adds: It's certainly not an intuitive button; I clicked it to set up the account the first time I ran iTunes4, but I hadn't thought to click on it since...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030429090050857