Can the iPad stream music directly to my AirPort Express?

Not as of OS 3.2, no.

The iTunes interface on the iPad is provided by the iPod application. This is not iTunes and has a much more limited feature set.

Whether Apple plans to provide the ability for the iPad to stream music directly to the AirPort Express is unclear, but it seems unlikely, given that this feature has yet to appear on the iPhone or iPod Touch.

One aspect that might have prevented this with the iPhone or iPod Touch was battery consumption. The iPad's has a greatly increased battery capacity might leave the door open for some possibility of this feature coming in the future.