Since the Spannish GP, there are rumors that Williams and Cosworth could divorce at the end of the season, and that Toyota are eager to provide engines to the team.
http://www.grandprix.com/ns/ns16848.html
I found the arguments advanced here quite surrealist, Toyota would be motivated to be motorist for Williams because they fear to be beaten by a customer team. sounds a little childish, plus I don't see where is the shame for a five years old team to be beaten by such a name as Williams, even with budget in mind...
But on the other hand, if Cosworth engines keep up blowing, I wouldn't be surprised that Williams decide to split ways, as they know long terms developpement will be an issue for Cosworth. If really developpement is a lot slower for them than their competitors, they should have started with a reliable basis, even if that meant being less ambitious performance wise.