An iPhone or iPod Touch can do it. Kingston has a remote than can be plugged into the Express. If you don't have an iPhone or iPod Touch, I would suggest getting the Griffin AirClick USB, as it uses RF (so no line of sight). It also works with other applications, like PowerPoint and Keynote.
Don't forget you can also use a bluetooth keyboard, which is what I did between the time my Linux friend broke my AirClick while developing drivers for Linux, and the release of the Remote App for iPhone.
TEG