This is when an app is tied up doing something CPU-intensive. There's just plain no way around that one (though the app "should", when possible, be doing its work on another thread so that the UI stays responsive). If there were several more apps in the background all further tying up the CPU, it'd be way worse.

My 3G is unresponsive regardless of what app is running or whether an app is using the CPU. Even the springboard locks up at times. It's the OS itself (or the 3G hardware, more likely) that's the problem.

Not saying that multitasking wouldn't make things worse, but don't give any props to iPhone OS for avoiding delays. OS 3.0 is embarrassingly laggy on the 3G.