I'd like to know if the following is easy/possible/recommended.
The version of Ubuntu I'm using (jaunty) has an out of date package that I need to upgrade: http://packages.ubuntu.com/jaunty/libapache2-mod-wsgi (it's at version 2.3).
A future version (karmic) has the new version: http://packages.ubuntu.com/karmic/libapache2-mod-wsgi (version 2.5)
Is it possible to use apt-get to install the new version in a way that will work? Or is it simpler to just install from source?
As I understand it, if I install from source, then I'll be responsible for keeping it up to date, and apt-get update/upgrade won't do anything.
If I add the karmic universe repository to my package sources, what will happen? Will it affect any other installed packages or just the one I add? Will apt-get install libapache2-mod-wsgi get the right version? What happens if/when ubuntu update the jaunty package?
One uses "apt pinning" to do this. An example is given here http://osdir.com/ml/ubuntu-users/2009-06/msg00516.html, you'll have to review the man page and adjust this to your needs.
You can use non-standard packages in Ubuntu. You have to take care of the dependency resolution yourself.
You can download the package from the link in your questions.http://packages.ubuntu.com/karmic/libapache2-mod-wsgi
After you download, you can install with sudo dpkg -i or gdebi the gui .deb installer.
You may have to download additional a packages to resolve dependencies.