Actually, there've been significant rumblings that she might indeed be back (in a guest-starring role) sometime later this season... I believe SFX Magazine (among others) mentioned that.
Since the recent interviews indicate that she enjoyed working on the show and wouldn't be adverse to reprising her role (in a limited manner), and since J.J. Abrams has stated he'd love to have her back, I think some hurdles have been cleared.
We'll see.
btw, I'm also a recent convert to the whole Alias scene -- got hooked when I finally Netflixed the first discs of Season 1 about a month ago, and then my wife and I jammed through all three seasons. We got through the first season thru Netflix, then that got too slow, so we borrowed Season 2 and 3 from a friend. We've been watching 10-12 episodes every weekend (!)... and last weekend I managed to grab the Season 4 eps off BitTorrent, so we're finally totally caught up.
I didn't hate Season 3 nearly as much as some of the comments I've read here (and elsewhere) -- I think in part that's due to DVD: the show plays a lot better when you can watch through it in chunks, and you're less annoyed at a single episode because you can always start watching the next. And all the doom-n-gloom from other Alias folk prepared me for the worst -- it wasn't all that bad!

I'm having a blast so far with Season 4 -- if for no other reason than to laugh at Abrams' ballsiness of reinventing the show whenever he feels like it. Sometimes it works, sometimes it doesn't, but I sure prefer that than the staleness of watching the same sort of show endlessly. If season 4 turns out to be nothing but stand-alones, then I'll be disappointed, sure -- but every season, it seems, has taken a few episodes to get up to speed... and if I learned anything from watching B5, it's that seasons tend to start slow and build up once the new plotlines, characters, and situations get fully set up (both for new viewers and oldbies).
Tomorrow will be my first "live" viewing of an Alias episode. Woohoo!
