Be careful with OpenSUSE - they still use GRUB as their bootloader, while Ubuntu uses GRUB2. (As of 11.4 there is an option to go with GRUB2 however.) If you're using Ubuntu as your primary OS and replace GRUB2 with GRUB during an OpenSUSE install, it might cause problems. (At the very least, some of the command line structure is different, so it might make it hard to follow a how-to, for example.)
I haven't done an install with 11.4 but I do have 11.3 installed on a second hard drive. If they haven't made major changes, one of the options when installing OpenSUSE is to install no bootloader at all. After the install you need to run sudo update-grub from Ubuntu to add OpenSUSE to the bootloader. (This was the option I went with, since I wanted to keep Ubuntu's bootloader.) The only downside to doing it this way is that whenever OpenSUSE updates their kernel, you have to update the grub manually (from within Ubuntu - at least that's the easiest way IMO).
I think Fedora 15 uses GRUB2 as well; if you install Fedora second you'll get a Fedora theme for the bootloader but there's otherwise no difference.