While playing the game if you ALT+TAB out then sometimes the game never returns to full screen mode. Also for some of my friends the game never switches to full screen.

After some digging with Google I found the following fixes that worked for me and my friends:

Fix #1:
Simply select the game window and press ALT+Enter
This usually works.
Try both the left ALT and the right ALT buttons.
Some users had better luck with left ALT+Enter.

Fix #2:
If Fix #1 is not working then you may have another application that prevents full screen mode. In my case I had TeamViewer installed and running. Quitting TeamViewer allowed me to swtich back to full screen mode.

Fix #3:
This game runs using DX10 by default and running it with the older DX9 seems to help some people.
Fix submitted by ThameHame:
Go to the folder where you installed the game, and there should be a launcher called "AssassinsCreed_Dx9". Double click on it, and you should be able to launch it on dx9. To make it easier, just run a shortcut and move it to the desktop.

Fix #4:
Open NVIDIA control panel and go to resolution. Make sure the resolution is set to 1080p (or other 16:9 resolution) and NOT 1080i.
This fix submitted by Reeces_Pieces

Fix #5:
Go to the Assassin.ini config, and set your resolution and other graphical settings if you want, but most important: set Fullscreen=1.
This fix submitted by Threatz

Hope this helps someone.