I'd HIGHLY SUGGEST that the upgrade notice BE REMOVED until more bugs are worked out. PLUS it should not say PLEASE UPGRADE.
It should say "Upgrade is currently rated at 43% success"
or some such thing. It should also link to a forum that
explains the DANGERS first.
Sure it always says "backup first", but do you know how many people know how to use backup files? Personally, I've never managed a successful backup after 5 tries.
So here is how I successfully recovered.
I deleted all my sites hosted files except for
-wp-content folder and
Then I FTP'd up the new 2.8 files except for the wp-content folder.
After that, going to the site resulted in a database upgrade button.
Clicking that and the site was up and running.