With Ubuntu you can upgrade your distro online when a newer version comes out, such as 8.04 to 8.10, without having to reinstall. Are there any other distros that can do this? For example, I installed OpenSUSE 11.0 and attempted to upgrade to 11.1 using that package manager but it just hosed my entire system, and then I read that you would have to reinstall the OS to upgrade.

Edit: it would be nice to be able to downgrade also.

With Debian you can upgrade from the cli.
Change your /etc/apt/sources.list to the new version.
Then aptitude update && aptitude full-upgrade.
Once upgraded it is not possible to downgrade to a previous version.

Regards
The trooper.

is my preferred way

Just edit your /etc/apt/source.list and put the new version in there. If it used to be fiesty, change it to hoary, or whatever the Ubuntu distro's are called. I'm a Debian guy so I don't follow the naming of Ubuntu, sorry. You should find the upgrade instructions on www.ubuntu.com though.

Where did you read that because its possible to upgrade openSUSE using the package manager or via cd/dvd. You need to remove your openSUSE 11 software repositories and setup those for openSUSE 11.1 (or simply change 11 to 11.1 in the repo config section in YaST) and then do a dist-upgrade e.g.

Also you can do it with Archlinux, Sourcemage & Gentoo. I think Slackware can do too, but sometimes breakage may occur with version upgrades.

Pretty much most modern distributions can be upgraded in a live environment if you're careful. You can even change distributions live if you're very careful (I don't recommend it). That being said I've had the best luck upgrading CentOS/RHEL, Debian, and Ubuntu in the past. No matter what distribution you're using you should take special care to read all the notes about performing the upgrade and be aware of any special sticky spots.

Thanks for the informative replies guys. I've now tried Gentoo, Madriva, PCLinuxOS, Ubuntu, Debian, & OpenSUSE.

I don't remember where specifically I read that. I've gone through dozens & dozens of pages working on this kind of stuff. I attempted to do exactly what you said with OpenSUSE, and through some kind of dependency hell ended up having to upgrade my KDE version, which required an upgrade to a different package, which needed a different repo also, etc etc. until I ended up with a system that couldn't load the package manager but could get online, and the next time I tried to work on it the system wouldn't even boot, and that's where it's sat getting dusty.

Or, you can always go with my personal favorite:

wget http://ftp.slackware.com/slackware-current;

upgradekg --install-new ./*/*.tgz

Sidux via the smxi tool updates/upgrades flawlessly.

Gentoo can do this blindly.

In fact it's not only that it can, but that there's no other way to upgrade. Gentoo has no versions. All that a new version brings is an updated livecd that you can use to install Gentoo. But once you install, emerge --sync and emerge -auDvN world you get the latest.