Confirmed bugs: Facesuck seems to be totally whacked in Ion mode (it works fine in Baseline only mode). IonPower passes all the JIT tests, though, so this must be something that the test suite does not cover. I'm investigating some other conformance test suites and corrected a couple other variances so far between Baseline and Ion but none of them appear to be what's ailing Faceblech yet.
Also, we have another web font that makes ATSUI puke, except it has an inconveniently null PostScript name so we can't filter it with the existing method. Fortunately Tobias had come up with an alternative font filter system some time ago that should work with 10.4.
Not confirmed (yet?): a few people have reported that memory usage skyrockets upon quit and the browser crashes (inevitably after exceeding its addressing space), on a variety of systems, both 10.4 and 10.5. I can't reproduce this on any of the test machines.
I need to do more looking into the stored passwords question.