We already knew about the game, but it turns out that Crawfish Interactive has the development duties, not Capcom.
Although some websites (including our source for this article, FGN Online) are treating this as a game announcement, we actually learned about the GBA version of Street Fighter Alpha 3 back at the E3 Expo in May. "S-U-P-E-R" Ty Shughart has actually had a preview up for the game for weeks now, where you'll find tons of info on SFA3 and what the handheld conversion might hold for it.
Anyway, Crawfish Interactive was also behind Street Fighter Alpha: Warrior's Dreams for GB Color, so they do have experience with the series. The previous game was considered to be a very good port, although the lack of two-player support hurt it greatly. You can bet that they'll be fixing that for this new GBA title. The game is essentially a port of the Dreamcast version, with all 34 (!) original characters, plus three new ones just for GBA. It will hit stores this October.
Here's what Mike Merren, development director at Crawfish, had to say:
"We're delighted to be working with Capcom again. The StreetFighter Alpha 3 project follows on from our work on StreetFighter Alpha: Warrior's Dreams for Game Boy Color. If Nintendo has moved handheld gaming technology ten steps forward with Game Boy Advance, we have to make sure that we move the quality of the game 15 steps forward in response. The Game Boy Advance is an incredible console. We expect StreetFighter Alpha 3 to be a incredible game."