Interesting. It didnt occur to me that a functions.php problem would wreck the whole install, but I suppose that makes sense.
You’re on the right track – here’s what I’d do: Get into your site with an ftp program (or cpanel, I suppose), and either delete or rename functions.php (it will be located in wp-content/themes/whatever theme folder), or just rename/delete the entire theme folder. Once you do this, the frontend of your site wont display, but you should be able to login to the backend again, and change the theme back to default.
After all that, if you still want to use that theme, we can probable get the function order sorted out.