Random idea:
Get the current time in a JS block in the head, before you load any CSS and JS, and compare it to the time when the dom ready event fires. If there's no real difference, load hi-res backgrounds and so on. If there is a real time difference, don't.
Wouldn't that be measuring latency more so than bandwidth? You'd run the danger of confusing a satellite internet connection (high(ish) bandwidth, high latency) with a third-world, low bandwidth connection.
Satellite ISPs have low data caps and/or charge a lot per GB of transfer. Avoiding unnecessary downloads seems like the correct behavior in this case.
I think the best solution would be an optional http header. That way, the server could choose to send a different initial response to low-bandwidth users. If connection speed is solely available via JavaScript API or media query, then only subsequent assets can be adapted for users on slow connections.
