Easy to do in InDesign, but not so much in Illustrator. It’ll probably be in CS6 (though I’ve been thinking that for quite a few versions now)
Anyhow, it actually is pretty easy to do in Illustrator, just not as straight forward. You need to use the power of the appearance panel (which you can read more about here, here and here) and you won’t technically be putting a gradient on a stroke so much as you’ll be making a fill behave as a stroke and giving that fill a gradient fill. Yeap.