gstassrender for gstreamer (Totem) is coming along quite nicely. Embedded font support just landed a few seconds ago (thanks slomo) and font scaling shouldn't be too far off.
By early next year, Linux users will have a hassle-free, out-of-the-box alternative to mplayer-build for VSFilter-equivalent subs.
It's a really a shame that things had to come to this, but the mplayer trunk devs really don't want an up-to-date libass that pulls from an upstream library.
Edit: They plan to use Cairo for scaling in case you're interested.