I was able to force the update today for my Rogers Lumia 900. Here's how I did it, based largely on readings here at wpcentral:
Originally Posted by Nemesis-X
1) put phone on airplane mode -- WiFi and cellular data disabled
2) disable WiFi on PC/laptop
3) connect to PC/laptop via ethernet cable
4) connect to Zune, allow connection to phone
5) click on "Phone", then "Settings"
6) click on "Update", and about 1 second after the search for update starts yank out ethernet cable
7) phone displays that update is available if you have timed it right (I assumed it would be the PC informing you but it was the phone itself)
8) reconnect ethernet cable to PC/laptop
9) perform Zune "Update" again and you'll see the update to 8112
10) follow the 10 step on-screen update process for 8112
11) the last step of 8112 update is a scan for further updates which will find 8773
12) follow the 9 step on-screen update process for 8773 (you don't have to "force" this update)
13) you're done -- the whole process probably takes 50-55 minutes
Hope it helps. Happy updating to those who don't want to wait for the natural roll-out to reach them.