I think about it. But it was discovered that for specific application there is no advantage because it looks like N900 on high frequency uses less energy per CPU clock.

So, underclocking may help only if you have some application which eats CPU without any useful finite work - any useful finite work requires approximately the same number of clocks in any frequency.

And yes, there are some another very specific scenarios then underclocking may save battery even more energy expensive CPU clock.

EDIT: modest can be set into background, I tested it. Moreover, almost any application can have nice 1 priority - that actually has a VERY small difference with normal priority.

"Can the N900 detect when I'm overwhelming a EDGE network and bump me up to 3G, and then drop back down to EDGE when the network traffic slows down?"

it has no sense - measurement shows it uses the same energy amount. 3G was designed for low power device in mind. I can't give you a reference for a moment but somebody did a measurement.