His presence at Brands Hatch alongside Mercedes is apparently linked with at least two prospects: Williams F1 looking for a new engine supplier or Nico Rosberg being in advanced talks with Mercedes for switching to McLaren next year.
Although the 2010 Formula One entry list clearly sees Williams still partnering Toyota on the engine supply market, it is believed Sir Frank Williams is currently looking for new power plants. The latest rumors suggest, however, that the 67-year old is more close to inking a deal with former collaborates Renault, with whom he won plenty of championship titles back in the '90s.
As for Rosberg, it's quite clear that Mercedes are currently courting the 24-year old driver for a multi-year contract alongside Lewis Hamilton starting next year. Rosberg's deal with Williams runs out at the end of this year, and he suggested plenty of times during the course of this year that he's considering all options for next year.
Heikki Kovalainen, Hamilton's current teammate at McLaren, had an extension option with the Woking based team that expired at the end of July. That means Mercedes are not too keen on keeping the Finnish driver from 2010 onwards also.
Rosberg was offered the same position with McLaren in the past – at the end of 2007, when Fernando Alonso parted ways with the British team – but turned it down for a 2-year extension with Frank Williams team.