It could be a classic corporate manoever for Merc to pull out of McLaren and put their eggs into BrawnGP. They already own MBHPE Ltd which designs and manufactures the F1 engines and KERS. If they want to switch their works involvement from McL to BrawnGP it will probably cost them nothing and would perhaps even make a profit. Shares in Brawn would be comparably cheap and some money would come from selling McLaren shares. The biggest profit would come from not subsidising McLaren at a tune of hundred of millons €€/annum.
I think this was talked about after Australia, and I continue to see it as a good move for Mercedes.
Not only would they gain the early entry cost benefits, but they would get away from the seemingly endless controversy that surrounds the McLaren team.
Who knows? They could still sell engines + KERS to McLaren, but by moving their 40% shareholding from McLaren to BGP, they could seriously be inline for a dominant future in F1.
Especially with everyone else bailing out for next year!