Merlin Mann is doing a series on keeping your inbox level down. I like what he has to say about leaving a message in your inbox without acting on it / filing it / deleting it:
Just remember that every email you read, re-read, and re-re-re-re-re-read as it sits in that big dumb pile is actually incurring mental debt on your behalf. The interest you pay on email you’re reluctant to deal with is compounded every day and, in all likelihood, it’s what’s led you to feeling like such a useless slacker today. Maybe? Think about it.