The BRZ is actually a Toyota/Subaru love-child. Toyota built the FT-86 concept quite a few years ago and the successor to the AE-86 Sprinter (Corolla) that is a cult classic in Japan.
I don't know at what point they decided to share the platform with Subaru, but the low centre of gravity they could achieve by using a boxer engine was a factor. So, Subaru went ahead and developed a brand new flat four which will début in the Subaru BRZ/Toyota GT-86/Scion FR-S.
I'm very curious myself what the performance envelope will be on this car... it's got less hp than the WRX or STi since it's normally aspirated, but it's substantially lighter with near 50/50 weight distribution. I think the BRZ has it's sights set squarely on the MX-5 personally. Nearly 30hp more than the Mazda, and less than 200 pounds more I believe.
TRD and STi are already working on parts for the car, so there's probably going to be a nice go-fast catalogue ready by the time it makes it to showrooms.