[prawn]

I've always imagined that a window washing robot would have a motorised internal component and a magnetically connected external piece. You'd put it on a window and let it go. If you walked past and noticed it not moving but still charged, you'd swap it to another window.

Obvious flaws (need two people to set it up - one inside and one out) but might work.

How else could you do it? Couldn't trust suction caps on a dirty surface.

[veidr]

Yup, I've always imagined two magnetic components, too. Two magnetic robotic butterflies doing a beautiful dance of cleansing.