There is an active thread of LMDE users working towards LMDE specific theming: viewtopic.php?f=141&t=64522
Debian testing had several gdm3 updates prior to Squeeze going final. I think the Mint devs may have had the overrides on page 2 of the thread linked to in my previous post to keep the backgrounds related to Mint. So far no backgrounds/wallpapers had been published by the team specifically for Mint. So probably when the latest Mint tools came out, they pulled in the latest LM 10 wallpapers.