I think slumming is the wrong word. If it were Winslet insisting on the placement for own self-promotion, it would indeed be slumming, but this is all Harvey.
Given that Winslet's husband is the director of her "other" film, it is more likely that Winslet would probably prefer to be nominated for that film if she even thinks about these things. However, early word is that DiCaprio is the standout in Revolutionary Road, so it may not even be an issue. On the other hand, Harvey's Oscar whoring for The Reader is so blatant it leaves such a foul taste in everyone's mouth that unless it bowls over critics and audiences like nothing since (fill in the blank) it may not be nominated for anything and Winslet may have to wait for that hard to get sixth nomination that should put her in the winner's circle when she does get it.
In the meantime if I were a betting man I'd put my money on Kristin Scott Thomas in I've Loved You So Long.
“Life is a shipwreck, but we must not forget to sing in the lifeboats.” - Voltaire