That depends on what they want to emphasize. 16:9 is cheap because HD video is 16:9. If you're viewing HD video, it's perfect. If you're authoring video, it's not. You'd like some space for the editing interface. Apple, of course, has skin in that game.
Apple has been using unorthodox screen sizes since the Titanium Powerbook's odd 3:2 ratio. If they cost more? Apple's a premium brand. If Apple does adopt 16:9 it will be on the consumer machines only. The professional machines can soak the extra cost, especially in the quantities that Apple orders displays.
With the iMac, I think Apple will avoid that being a Pro machine as far as possible. Meaning, 16:9 for viewing, not authoring HD video. iMac will be as consumer a desktop as possible, in Apple's eyes. As for the MacBook Pros, I think those are safely 16:10 for another year or so, while the MacBook white may be 16:9 in this coming revision. Wait, scratch that, I think besides the iMac no other displays will be 16:9 for another year or so.
The one problem is that while they're prevalent in in the TN ranks, you don't see a lot of IPS displays with a 16:9 panel yet. The only one can think is the brand new 23" from NEC. It does come with a pretty considerable price decrease versus 16:9 IPS screen though. Then again, its lower resolution than the new 23" TN screens.
I think Apple will not use IPS displays in their iMac 16:9 LED backlit panels. IMO. Apple could be offsetting the cost of LED backlighting by using purely TN panels.
All this speculation is making my head hurt. I'm getting future hardware overload after this past few weeks.