If "learning" linux is the goal then I suggest slackware. But there is no live cd for this.
Here is direct link to a download list of the latest version, if after using a live cd you decide you would like to install a linux distro:
But live cd was your request. There are many great distro's out there. I suggest you try a version that offers kde, kde is similar in look and feel to mac osx.
Debian is fairly old, almost as old as Slackware. Old is good in my book. Debian offers a live cd.
Ubuntu is based on Debian. Some say it is more user friendly than some other distros. Thats for you to say.
Kubuntu is well ubuntu with kde. They have a nice walk through at there site. Nice to get an idea of what its about.
Linux Mint is based on Ubuntu. I run this one myself. I picked this one to install on my fathers machine after his windows os gave out again. He used to run Mac strictly. So I felt as an introduction to a foreign os, Mint sounded the best choice. 12 is nice.
There are plenty more. If you pick any one of the ones above, Im sure you will be happy with what you see.
If you need some more help making up your mind, check this out:
This article talks about the major distros of Linux. I read this among a few others before making my choice. There is no best distro, just a matter of witch one fits you best. What with over 300 various releases
you should find your match.
Feel free to post back with questions or thoughts.