Thank you for the response.
For any further readers:
This issue was not a wordpress issue, but a compatability issue with the latest version of the Headway theme. The fix was to temporarily remove the theme’s folder from themes (just moved it to the root directory), then trying to access wp-admin again. It should now work, finish upgrading the database to 2.9, then move the theme’s folder back into the theme directory and you should be good to go.