AirPlay and AirPort Express
The new Apple TV will allow you to stream audio, video and photos directly from an iOS device to a TV that is connected to an Apple TV, and thus by extension also to any speakers that are connected to your TV. I'm not particularly bothered about that added functionality, as I really only use my AirPort Express speakers to stream music. If you could stream audio directly from an iOS device to AirPort Express speakers, that would be a big help if, for instance, I can't be bothered to switch my MacBook on or if my wife wants to stream from her iPhone when I am away from home with my MacBook (on which is stored our iTunes library).
Equally I don't really want to shell out for an Apple TV for the privilege of streaming from an iOS device, and even if I did I would not want to have to have my TV switched on to allow my iOS device to stream to my speakers.
Very early days, I know, and a little hard to answer until iOS 4.2 comes out, but if anyone has any preliminary thoughts I'd be interested to hear from you.
MacBook Pro (mid-2009); MacBook (early 2008); iPhone 3GS, Mac OS X (10.6.4)