I posted in the other thread so I'll do it here as well. The Lock Manager app that WPC reviewed updates Bing to today's picture(I've always had the issue of the Bing BG being from the day before). Those of you having issues with it updating at all may want to try and use Lock Manager and see if that helps.
I noticed this same problem. For me, the solution was Amazing Weather HD. Unfortunately, the price is $1.99 for those of you that do not like to pay for apps. However, if you're willing to pay for this great weather app, they have a lockscreen function. It displays your weather AND you can select the background as the Bing image of the day! It changes every day to the correct image. Again, I know $1.99 might seem like a lot, but the app is great, has a great UI, and various live tile options. The lockscreen gives you current weather conditions, Bing image (or other background options), in addition to your normal date, time, unreads.