I’m sure this has already been heavily blogged, but I just noticed a new option in my “speakers” pull down available on the bottom right of iTunes if you have an Airport Express.

So effectively, probably according to bandwidth restrictions, you can have the song playing at your computer in one room, and in any number of external Airport Express locations.  You get a popup which displays a checkbox list of the speakers currently being broadcast to, and allows you to select more:

If more software / hardware companies thought through these seemingly insignificant additions, we’d have a life with better built software.  Simple is definitely harder, but better in the end.