In Windows, the hiberfil.sys sits right next to the pagefile and I don't think anything ir erased from it when you start up.. So, yeah.. I'd guess the file contains the contents of your RAM since the last hibernation. (Your pagefile contents are also kept intact between Windows sessions BTW (unless you ask Windows to clear the pagefile every time)).
Linux (and probably other unixoids) use the swap partition/file to store hibernation data so parts of it will probably be overwritten as you use your computer, but no guarantees of course.