I've just found out that you can cut a Nikon K3 focusing screen down to fit the D40, and it is very easy to change. You don't need to make the little tab - you can just cut it down to a plain rectangle. The K3 is the $30 screen with the split-image rangefinder prisms. It improves manual focusing and has no effect on any other functions. I can provide more details if anyone wants them.
Very interesting. And at $30 a lot cheaper than the KatzEye option. Add a small fee for cutting and you have a nice business idea there.
Fitting a manual-focus screen is a great idea though. I know plenty of people focus manually with D40s, Rebels etc, but for me almost any dSLR is horrible for manual focusing. Guess I've been spoiled by film cameras. If I were in the Nikon system I could definitely see myself using the D40 with a split-image screen as a manual-focus backup, or for specific uses (for example I'd much rather use manual focus for macro). This little modification would be a great improvement IMO (if the camera can't autofocus with autofocus lenses then why not make the most of manual focus?)