Try adding:
AddType x-mapp-php5 .php
to the top of your WordPress .htaccess file.
I tried to do it before posting this question to this forum and it messed things up – I was getting an error.
I had a similar problem of the upgrade just churning on endlessly without any error or progress message when trying to use the dashboard upgrade link. When I did some reading, it turns out that the MySQL version my database was in is not supported by the latest version. That is, my database was in MySQL 4.0 and now 5.0 is required. I ended up having to do an import of my old database into a new one set up with 5.0 and then edit wp-config to reflect the new database info. After all of that, I just went and did the upgrade manually via ftp rather than trying to use the built-in upgrade, so I can’t say for sure that it absolutely fixed the issue. Nonetheless, with a few side issues probably not related to that (like posting being broken from the “new post” page), I am indeed now upgraded.
