Let's recount the bands whose names kept getting mentioned as possibilities (and now are mostly out)
Smashing Pumpkins- NOT playing SS according to CityBeat. I think they're booking their own headlining tour.
White Stripes- They are playing Viejas concerts at Bayside on Sept 18 according to their website and therefore are probably not also playing Street Scene a week later, unless that date is posted to throw everyone off.
Nine Inch Nails - I heard a rumor from a "friend of Trent Reznor's cousin". Considering NIN will be in Australia the week before Street Scene, I'd guess they're NOT playing.
Beastie Boys- After a full European tour all summer, I don't think San Diego is on their list of US cities to hit. But who knows. They just played the Croc in Seattle, after all.
Muse- Confirmed according to a band email that went out to fans.
Ozomatli - Confirmed according to Pollstar.
G. Love and Special Sauce - Since I don't remember where I heard this, I'll say probably in, seeing as he plays here pretty much every year. Plus, his buddy Tristan Prettyman lives in town, so he'll probably come and hang out and have her play onstage with him. Oh, and cuz he's touring with Ozomatli.
Slightly Stoopid- Because they're touring with Ozo and G Love in August and they can be considered "local" so LiveNation doesn't have to look any harder.
Justin Timberlake- JT just announced a huge arena tour. 9/5, Vancouver; 9/7 Portland; 9/8 Tacoma; 9/10 Sacramento; 9/12 San Jose; 9/16 & 9/17 LA...If he's not coming to San Diego for Street Scene, he'll probably be adding a show here close to that date. I kinda hope so.
Cold War Kids- Confirmed according to their MySpace page, an e-mail newsletter, and this page. Notice that the 9/18 show at the Embarcadero is not on this list. I really don't think that show is confirmed, especially when Jack White hears about his noise limitations on the bay.
I also have a list in my head that is completely speculative (and wishful thinking) and as often as I've checked all their sites, the Street Scene dates would work with their summer touring schedules. Those include Arcade Fire (playing Austin City Limits on 9/14), Flaming Lips (playing Monolith Festival on 9/14), The Kaiser Chiefs (Playing ACL), Clap Your Hands Say Yeah (playing ACL).
Some other hunches that I've looked less into but could fit according to currently posted tour schedules: The Killers, Ben Kweller, Spoon, Paolo Nutini, My Morning Jacket, Queens of the Stone Age, Augustana.
There are a ton of other bands I'd like to see, but I'm gonna end my guessing here. If I hear anything else, I will post. I'm going to give Street Scene a go with LiveNation if I can get a presale pass tomorrow, but it's every person's decision to make for him/herself. I assume the official preliminary lineup will then be announced next week or the week after, and if sales are strong, Street Scene will have negotiating power to bring in more mid-level bands, so don't wait on tickets. I've said all I can say about this for now. Good luck getting tickets tomorrow at 10 am.