Think iPhone-esque pincer commands, but on the ever-so-slightly chunkier G1 by T-Mobile. Apparently Android allowed for multi-touch all along, but the way the OS is structured, multi-touch events were obscured. Independent developer Luke Hutchison has found a way around this with an app (open-sourced on his website, no less) that modifies a single Java system library.
There are some limitations, but it works pretty well on apps like Google Maps, where scaling the map in and out seems to be have been designed with multi-touch in mind. If you want the programming nuts and bolts behind all this, Hutchison goes into detail on his site, where he also has a demo video up.