In addition to my previous post, I think it is useful to prove that my installation of Windows 7 has its function "hybrid sleep" really enabled. So, I made a special test : after using Win7 "Sleep", I cut off the power supply then I restored. I restarted on my Ubuntu 12.04 in order to check the state of my Win7 partition :
head -c 8 /media/Win7_partition/hiberfil.sys | od -t x1
0000000 48 49 42 52 09 00 00 00
We can see that the "hybrid sleep" is truly enabled and the hibernation state of the Win7 partition is detected correctly by Ubuntu 12.04.