Really strange error - I've upgraded one of my older blogs from 1.5 (it was using that Multi-user hack so I wiped everything except the themes and plugins) to 2.7.
Upgrade went fairly well, had some permission problems hence the whole scrub then reload...
All is well, even the themes amazingly, but I can't login to my Dashboard - I get a 500 Internal Server error. Weirdly though I can then goto the blog and click on edit and get into it that way - just wp-admin/index.php isn't working.
Disabled the theme to classic/default as it comes with 2.7, disabled plugins, deleted the old .htaccess, reinstated and then deleted it and created a new permalink...none of these fix this.
It's very weird. The server is now using PHP5.2 and I'm on 2.7.1 - I run other 2.7 blogs on the same server/MySQL/PHP with no problem, and others with PHP4.4 - same setup though.