I was thinking, since on my n95's battery last significantly longer in 2g mode, what if the n900 would default to 2g when there is no browser, internet radio or other network intensive application open? In standby mode there are only the wigdets, but they transfer so small amount of data so the lower speed is not issue. When the user requires a fast connection and opens, for example, the browser, n900 would then automagically switch to faster 3g connection. I thought this could improve the battery live greatly.

Here's a quote from a finnish forum confirming that the 2g standby time is a lot longer than when using 3g

So, I think this would be a pretty neat "official" feature, but if not, someone with programming skills could create a daemon or something to do the job.