A workaround is to create a second Administrator account, then use that account to change ‘admin’ to an Editor, save the changes, then back to Administrator, and save the changes again. That should cause admin to appear in the dropdown list.
Also see http://trac.wordpress.org/ticket/2818
I had the exact same problem, but Bobcat’s workaround is an excellent one – fast, easy, and works. Great suggestion, Bobcat!
The bug is marked ‘worksforme’ but sadly it doesn’t work for at least some of us.
Still, at least there is a work round. I have five blogs and growing, so what would be a small change in a single bog becomes quite a bore with several. However, witout the altruistic people responding to queries in the forums we would be completely sunk! Thank you.