Why am I getting a "Connection Information" page in WordPress 3.0 installation?

This is perplexing.

I have just set up two new domains on our family webserver, both of which I have installed with the new WordPress 3.0.

I'm already running older WordPress installations in other domains hosted on the same machine, so I know about the setup in PHPmyAdmin and in how to install WordPress.

That said, for some reason, on one of the domains I get a Connection Information page asking me for ftp credentials, etc., everytime I try to upload a new theme or plugin, whereas on the other domain, which underwent an IDENTICAL installation process, I have had no problem using the upload feature to upload new plugins and themes.

I have completely dropped the database from PHPmyAdmin and cleaned out the root directory of the problematic web folder and done a fresh install, but keep coming to the same problem.

Any ideas on what may be causing this?

One more thing, when I do enter in the server's FTP credentials, it doesn't accept them and just returns to the same page asking for the creds, as though the data I entered was incorrect. (Yes, I've even tried setting up a new FTP user ID, but no dice.)

Thanks in advance!

Sara