We (Mozilla) now have automated layout regression tests running on tinderbox. The good news is that 51 fixed bugs in the layout components have tests checked in and 11 more are marked as not needing tests (code cleanup bugs, basically). The bad news is that this leaves 6967 fixed layout bugs for which we either need to create tests or decide that tests are not needed.
Which means that we desperately need people writing tests. Now the beauty of reftest is that you don't need to know anything other than HTML and CSS to write tests, so pretty much anyone can do it. There's a tutorial on doing so. While the test suite can't be run in shipped builds at the moment, running a single test is not too hard: load the test and reference in two tabs and switch back and forth. There should be no differences (and ideally there should be a diffence in the old, buggy build).
If you do write a test for a bug, please put "[checkin needed]" in the status whiteboard and flag the bug as "in-testsuite?" so we can find the tests!