I do strongly believe that Ursa is dead. Sad thought, yes, but there's too many hints.
"The Southern Raiders" is meant to be one of the saddest episodes in the series, right? It's meant to be about Katara's mother, right? She's on a ninja mission with Zuko, correct?
Ursa hasn't been mentioned once since Ozai pulled the whole treason story out his ass told Zuko what happened that night...
I came across a little gem for this theory on AvatarSpirit.Net. I won't hotlink, but I'll give you the image number, if anyone's interested in looking. It's "NYCC Season 3 Finale Trailer" in the screenshot section, Image 034. Ignore Katara's rapeface and look at Zuko, he looks lost and miffed. =( It's the same with every image of them, Katara looks like she either wants comforting/wants to comfort Zuko/both? Does anybody think those images would be from The Southern Raiders, because idk...
Ray of hope for Ursa from me is in Sozin's Comet, when it mentions "Some of Zuko's family" being at his coronation... How many members in Zuko's family are left, srsly? Ursa and Iroh are the only two I can think of left not proven dead that would not hate to attend his crowning (unless Ozai and Azula crack it and attend, waving at him like two geeks, which I can actually so see happening).
Yes, I do realise that it's almost 4 in the morning and I'm thinking in depth into a children's cartoon, stfu g'nite. =P