Well, Jobs & Co are known to use their very own builds of Keynote, and you usually see new transitions and build effects in their presentations before they show up in the next version of iWork.
So, I guess we'll just have to wait for iWork '10 or iWork X or whatever it'll be called. Rumors about its release has not been very consistent, but some say it'll be later this year rather than sooner. I hope they're wrong, as I want it NOW! Maybe it's as good as ready, and Apple just want to let the iPad have its shockwave before communicating basic stuff.
But, if you know Motion or some other visual FX app, you can of course imitate that cool dust cloud using particles and stuff, render it out as a QuickTime movie with alpha channel (transparency), insert it in Keynote and combine it with text dropping in.