As of yet we don't know how the iPad will interact with the Airport Express. It would be nice if Apple let's it stream to the Apple TV and Express... but I find it unlikely.
If you don't mind having your computer with your iTunes library hooked up to the speakers the iPod Touch, and likely the iPad, have a remote control program that can used to control your PC based iTunes.
Another option would be to use a heaphone Jack based household FM trasmitter, such as the ones targeted to provide older model cars (with just radios) acess to devices like CD playes and iPods. There are even specially made devices for older model iPods. The quality will drop off just like any FM signal depending on interferance.
Apple should already have headpone adaptors to allow you to connect standard headphone jacks into the Ipod and likely into the iPad.
Other options may appear at or shortly after release date. If this a major point for you, hold off and don't buy until you have all the details.
