Well, you all summoned me, and HERE I AM!
Several posters have already given the right answer: Meryl has box office clout, and can open a film. DDL can't, unless it's a Spielberg film, and let's be honest, not even the Spielberg name opened 'Lincoln'. Abraham Lincoln himself opened that film.
Meryl Streep would be billed FIRST, Daniel Day-Lewis second. Even DDL knows Meryl is in a different league entirely, both for talent AND iconic status within the industry, and the world at large.
He would know his place. This being said, I don't think he would do a film with her. I don't dislike him, and I agree he is very talented; but I get the feeling he is a bit territorial. Insecure. He treated Adjani like shit, and generally seems to have an odd relationship with women. He seems indifferent to strong or talented women. Has he ever done an interesting film with a strong woman role opposite his? His female costars were Cameron Diaz, Winona Ryder, Madeleine Stowe. And he agreed to do 'Nine'... that tells you all you need to know.
Secondly, as for the billing thing, let's not forget: Goldie Hawn is a close friend of Meryl's. The reason Meryl agreed to third billing in 'Death Becomes Her' may well have to do with that.
She is not close friends with DDL, though.
As for Meryl being difficult: has only happened twice, with 'Remains of the Day' and during production of 'It's Complicated', where she had the cinematographer fired.
Finally, r39: Streep was as much the star of that film as Diane Keaton. As for overall fame, she's long eclipsed Keaton, so I don't know what you're talking about.
In any case, aren't we all looking forward to MERYL STREEP handing DDL his third Oscar in a few weeks' time? Yes, we are!
History in the making, with Meryl front and center, as it should be. ALWAYS.