Check this out:
Bright, natural but diffused light.
And since I wrote my reply to the article, I have started using a tripod... This allows me to use a longer exposure (without getting any camera shake) - and a smaller apperture to keep the whole painting well focused.
You can even get away with photographing the painting behind glass if you use a polarizing filter but I reckon you will generally get a better photo if the painting is not yet framed behind glass.