Okay, I've figured out what to do. Yahoo just made some changes that have affected the site's compatibility with BlackBerries and other PDA phones.
First off, bookmark any pages you use on the HTML version of the site (for me, that would be Yahoo 360). Next, from the menu of the BB HTML browser, choose "Go To" and enter www.yahoo.com
as the url. When Yahoo's homepage loads, it now comes up as the WAP version, because it now autodetects that you're on a BlackBerry and loads the WAP site.
From the WAP-formatted site, log-in, making SURE to check off "Remember my ID and password". Then, go into your bookmarks and use the bookmarks you made to get back to the HTML versions of the pages you need.
If you haven't bookmarked the pages you need, from the WAP version of Yahoo, BEFORE logging in, click the link for "More Services". That link takes you to the full HTML site. You won't be able to log-in from the HTML version, but you can bookmark useful pages, then follow the rest of the steps above from there.