I've just tried the cover on/off comparison with my almost new Desire (from Ebay, but it looks like the previous owner hardly used it). It's a subtle difference, but a difference all the same (caused by the fact that even brand new perspex will do something to the refraction). Colours are bolder without the cover, and there's an overall increase in sharpness. With the cover on, it's as if there's a thin mist overlaid on the photo.
It's a large file, but the differences are visible here (cover on-off, from left to right)...
http://www.sfocata.co.uk/misc2/desire_camera.jpg
(And that's after a 33% size reduction and 90% jpeg compression... the differences between the 1.1MB originals, viewed in Lightroom, are even more noticeable)
Any more news from the person looking into a bulk deal on glass replacements? Or are these watchglass suppliers our only hope?