I don't think I've ever gotten one for 1680x1050 - the Atlantean stargate one I have at the moment was a modification I did myself of a much bigger 4:3 ratio one from the SciFiWorld website.

1440x900 - I have found some I like on the internet, but from a fanart perspective, no. I almost never see ones posted on LJ that are in any form of widescreen, let alone this size.

Before I had the 1440x900 monitor for this PC, I had a 1280x1024 panel, and while I managed to find some fantastic walls in that size, again it was a size lacking in fannish wallpaper.