LG Optimus 7 with Telus, I had to force the update to Mango towards the end of 2011 since it wasn't offering it through Zune. Everything was fine until the apps went off Zune and to the new Marketplace site then suddenly I wasn't able to download apps anymore because windowsphone.com kept telling me my phone wasn't up to date. My guess is that it wasn't recognizing my update to Mango because I "forced" it. And as I understand you need Mango to use the new Marketplace.
I called Telus and best they could suggest was uninstall and reinstall Zune. Didn't work. So finally I tried the "disconnect internet" trick to force updates out of hiding and it took some farting around with the timing but it worked. I had 8107 so the first update went to 8112. Marketplace still wouldn't work. The I did the force trick again, and another update came available to 8773. Then I also removed and readded my phone from windowsphone.com. Finally was able to install apps. So far so good. Hope this helps someone else with similar problem since it's a little off topic, but very much related to the forcing 8773 OP- worked for me.