I happened upon a retail boxed 1GB ATi FireStream today and I decided to take a few pictures. The existence of this card was first noticed in the Linux drivers for a FireGL 7350 over at Beyond3D about a week ago and from what I've seen, their assessment was pretty accurate.
The card is indeed based on R580 with a board layout nearly identical to the FireGL 7350 including the 1GB of ram. The box I saw contained only this card and a driver cd which had been burnt and was labeled as being a beta. Also, I found the label on the CD interesting: "FireSTREAM Enterprise Stream Processor".
Judging from the similarities to the FireGL 7350, I don't think the card has any special hardware under the hood (with the exception of the fact that it uses R580 vs R520). However, the beta nature of the drivers suggests some very special software implementation at work here. My take on it: this is a revamped FireGL with new drivers geared towards making GPGPU (general purpose GPU) usage more efficient and/or easier to work with for developers.
I have to wonder if this new FireStream brand will replace the FireGL, or if ATi plans to divide their professional cards into two camps.