Only a total nerd would point out that Poison isn't technically a Street Fighter.
Regardless, the Mad Gear Gang member from Final Fight (and non-playable eye candy in Street Fighter III: Third Strike) is joining the Street Fighter x Tekken roster, as Capcom revealed today, along with a host of other characters. On the Street Fighter side, Dhalsim, Hugo, Cody, Guy, and Mike Haggar step into the mix, while the Tekken half of the roster confirms Yoshimitsu and Steve Fox.
It's interesting to note the presence of Hugo along with Poison, as both characters have only appeared in SF3: Third Strike. Perhaps this leaves the door open for other Capcom characters like Q, Necro, and Sean?