Well, I was pretty surprised when I was working on a bit of Syriac-related tomfoolery when I noticed -- completely caught off-guard, mind -- that Firefox 5 for Mac OS X properly renders Syriac OpenType fonts.
I haven't tested to see if it's also the case with other OpenType fonts with character joinings, but hey that's a step in the right direction!
Peace,
-SteveLabels: aramaic, Firefox, syriac