Seth's notes suggest some avenues toward breaking this DRM (beyond screenshotting, of course, which still works just fine!). This is a good avenue of research, possibly even worthy of a Mozilla variant optimized to circumvent the Google DRM.
If you wanted to write a proxy that would make Google Print pages capable of being saved to disk, you would presumably want to match
(although you'd need to be careful to match only the one in the definition of ".theimg", because it looks like there may at least one other background-image:url) and then replace
and somewhere nearby (I'm not sure how many tags up you'd need to go) insert a plain old
I haven't tried this because it felt like too much work relative to the previous two methods.
Contrary to what I expected, Google Print does not seem to check referer, so it seems to be possible merely to extract the URL from the definition of .theimg, and then to load it directly. Perhaps that will change in the future.