I think Ross Brawn approached his old paymasters at Ferrari, and I think they turned him away based on perhaps the timescales and payments needed... Also they might be cutting back in their engine department to concentrate on KERS, and supplying 3 teams like they did last year wasn't on the cards.
Mercedes have been fantastic in all of this, in publicly offering their services. Of course payments are needed, but Norbert Haug has been very gracious in helping out his fellow teams, clearly as he wants to see a full grid this year. It's even more gracious when you consider that Mercedes are supplying two teams for the first time in their F1 career, now it's three!!
Regarding the KERS systems, I was under the impression that Honda were developing their own system at Brackley, and whether this has been fitted to the Mercedes engine I don't know. I don't think KERS is in the engine deal. So maybe Brawn are going to race without it for a year before getting another deal or racing their own.
I assume though that on the engine front it's maybe a one-year deal, and Brawn should be looking at a long-term option on the Customer Cosworth engine that is due to become available in 2010. I think that this would be USGPE's (as they're now known) initial option too.