The #1 reason I still use post-its on my laptop and monitor (and... sometimes physically on my phone, in extreme cases): there's a real-world reminder that sticks around when you're powered off, disconnected, etc.

They're brightly colored, often aren't stuck on at right angles, and frankly refuse to be ignored. They're just the right amount of annoying, and I'll be interested to see how digital GTD applications manage to toe that line.