We've just got one of these http://preview.tinyurl.com/mh67at primarily for
kitchen floor tiles, bathroom tiles etc., etc. It says in the handbook that
came with it that it can be used to clean windows - specifically the glass,
not the uPVC, and it even comes with a squeegee attachment to do it.

I reckon if you squirt steam at a double-glazed panel it's gonna crack, my
missus reckons that it should be OK given that there's a specific
tool/instructions to do so. Who's right - cos I don't fancy paying for a new
window!

Cheers,

Pete