Attention Geocities users: If you're getting an invalid username/password error when publishing, this is a problem on Yahoo/Geocities' end. They have a note about it
here. Although that says it's only happening on new accounts, I'm getting the same thing on mine, which is years old. I get the same thing when I try to FTP there without Blogger, so I'm pretty sure it's not a Blogger problem. I've contacted them, and they say they're working on a solution. Hopefully it will be fixed soon.
Update: Pete Johns writes in, "I, too, have been suffering the login problem with Geocities (and I have had my account for yonks). However, I've found that if I change my FTP server to be 188.8.131.52 , then I can publish 'til my heart's content." It works for me, too. Thanks, Pete!