I too have an upgrade failure from 1.5.4 (or thereabaout) to 1.6. Here's a sample of the error :
1050: Table 'mdl_data' already exists
The first page of the upgrade went fine ... the second page threw up these errors ... I have been using Moodle for a few years now and was sooooo confident the upgrade would go smoothly - just like EVERY other upgrade I have performed - I (stupidly) didnt bother to backup !!! ( Ihave an older full backup which is about a month out of date so its not the end of the world ...)
Browsing the forum I see many many others have had similar problems ... most without much success ... anyway ... any suggestions appreciated.