Apple has a nice little utility called QuartzDebug in /Developer/Applications/Graphics Tools, and one of the features of this program is that it will highlight all redrawing that gets done in every application. Every time a view is redrawn, it flashes. This is a great tool for determining if you're doing unnecessary drawing in your drawRect: method, or if you're somehow triggering unneeded redraws some other way.
I've had a lot of people ask me about doing this on the iPhone. I've been told by several people that they saw this functionality demonstrated at the iPhone tech talks, which had me stumped. I completely forgot that you can get very similar functionality by passing an argument called NSShowAllDrawing to your application on launch. This is mentioned in this excellent technote on debugging.
Michael Fey has a (somewhat holder) blog post, complete with video, that shows how to use NSShowAllDrawing. His example shows using it in a Cocoa application, but the process is exactly the same for using it in an iPhone application.
Update: A couple of people pointed out in the comments that this option is also available using Instruments. If you run with the Core Animation template, there's a checkbox labeled "Flash Updated Regions", which is checked in the screenshot below:
I haven't been able to get the Core Animation template to work for me on the Simulator, though. I'm not sure if that's by design, or a problem with my installation.