Since 2.0 was released, I haven't been able to upgrade WordPress simply because I kept running into the "It doesn't look like you've installed WP yet." error. I banged my head against the wall until I stumbled upon the resolution.
This may have been covered in another thread, but I didn't find anything to that affect after a few cursory searches.
The logic in wp-settings.php I believe is incorrect, or at least should/could be modified. Specifically, I'm talking about line 121 where it states:
$db_check = $wpdb->get_var("SELECT option_value FROM $wpdb->options WHERE option_name = 'siteurl'");
The problem is, if you are migrating to a new location, domain, or a new server, option_name may not equal siteurl at that time. So, if a match does not happen, the user is left thinking the upgrade process has failed, when in fact it succeeded.
Case In Point:
I, like a lot of people, prefer to do development in a way that does not impact the front end. So, when trying the upgrade, I would create a new sub domain called "foo" and install my code onto it. After upgrading, the error would show stating that it had failed.
The above case would also be true if I was moving to a new server, and was accessing that page via IP (before DNS propagated).
I'm not a developer, so I don't know if that exception is needed for something else. But surely another sub-exception could be written to have a unique message stating "your upgrade has been successful. this site will display when accessed by $siteurl"
In any case, I think a number of users are struggling with this error. It might be worth making it a tad easier for those of us who are migrating.
My two cents.