There are 2 linux distros on my pc: an ubuntu 10.04 and a fedora 14.
If I suspend e.g.: the ubuntu are there any chances that I can boot into Fedora? Or that's impossible because the suspend uses the memory? What about hibernation?
This cannot be done with suspend because of the RAM issue, but it can be done with hibernate.
You will need to carefully setup your boot loader and make sure you aren't using any shared partitions (swap and wherever each kernel looks for the hibernate files).
You can even take this farther than just linux distros. I have successfully had a mixed linux / windows environment where I could hibernate and come back to any OS.
Practically speaking, virtualization has made my need for this obsolete. Using one primary OS, my need to test other distros or systems can be filled easily using VirtualBox without the hassle of partitioning and the brittleness that comes from everybody being able to touch the everybody else's stuff!