Comment 38 for bug 370200

2.6.28.9 vanilla also has working drm and agp. The last commit in linux-2.6.28.y that should affect this code was on 12/23:

commit 8960223d5973b3eaf8cbc1984969bb6c7661e7b9
Merge: d519c8d... 077ebed...
Author: Linus Torvalds <email address hidden>
Date: Tue Dec 23 17:01:40 2008 -0800

    Merge branch 'drm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/ai

    * 'drm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/airlied/drm-2
      drm/radeon: fix correctness of irq_enabled check for radeon.

So it looks like this problem is coming from a patch in the jaunty kernel...