Look at some of the local tradesmen's vans around you. When you look at them, see if you can work out what they actually do. Quite often you can't. Squeaky has a big yellow duck on his van - bathroom cleaning services or something!
I think as long as you have Window Cleaning clearly displayed, with your telephone number, the rest is just detail.
We are not a 'creative business', we're a practical business. When I have a plumber, I want someone whose sane, sensible and knows what he's doing. I guess when people are after a window cleaner, they want similar; but cheaper. 
Yep thats how I look at it.
There are some great sign written and vinyl wrapped window cleaning vans that I have seen driving around but most of the time the message on them is not clear to Joe Public.
Most people don't glance at your van for more than a few seconds,especially when you are driving along.
Try it for yourself.......I bet in a lot of cases when driving along or sat in traffic you don't give more than 2-3 seconds attention to other tradesmans vans.
The message should be short and clear.
In a lot of cases people recognise you from the association of seeing you in the area on a regular basis.
The words "Window Cleaning" or "Window Cleaner" and contact telephone numbers should be given prominence in my opinion.