We're seeing this error appear in the Dashboard in most of our custom themes after upgrading to WordPress 3.3.
Switching to the Twenty 11 theme seems to resolve the issue so this seems to be a backwards-compatibility issue.
The error message is:
Fatal error: Call to undefined method Custom_Admin_Bar::add_group() in /wp-includes/admin-bar.php on line 169
As a result of this, the Dashboard toolbar doesn't work in either the Dashboard or the front-end.