What?
Contributor License Agreements -- or CLA's -- are a fairly new phenomenon in the Open Source world. IANAL, but I am going to make an attempt to explain what a CLA is, and why it is necessary today, where it previously was not.

The most commonly used CLA is that of the Apache Software Foundation (ASF), and the purpose appears right at the top of the agreement:
"In order to clarify the intellectual property license granted with Contributions from any person or entity, the Foundation must have a Contributor License Agreement ("CLA") on file that has been signed by each Contributor, indicating agreement to the license terms below."
The agreement continues to in great detail explain that the person signing the document must both be the author (or hold rights or permission by the author), and be the copyright owner of the contributions they are planning on making to the project in question. It is clarified that the rights to use the contributions will always and forever remain with the contributor, and that the CLA simply extends this right to the project as well.

In other words, if you submit stolen code, you are breaking the CLA, but if you refrain from such follery, nothing's changed. You're still the copyright owner of your submissions, but you grant the other party these rights as well. Simple enough.

In the past, this has been a presumed agreement between the involved parties. If I help you out and patch up your code to make it run better, or submit documentation that makes your code more useful, or whatever, you and I both presume that I didn't steal the code from someplace and handed it to you in evil. This presumption is no longer enough, for good or worse.

Why?
CLA's became necessary as a direct effect of the SCO vs. Linux court case(s), which, summarized, are about: "SCO claims a bunch of contributors to the Linux source code stole that code without permission from SCO, and that thus, SCO is now the owner of Linux and may at their whim request that all users of Linux pay a royalty fee." Pretty scary, huh? Be that as it may, the court negotiations are as of this writing ongoing, but things are looking bad for SCO (for what it's worth).
But regardless, SCO's claim was a bucket of cold water in the face of the many maintainers of and contributors to various Open Source projects out there, as a legal matter was suddenly making things a tad more complicated. What if someone helps you out and gives you a bunch of really good, professional code, and what if that code is ripped out of some commercial, copyrighted very-much-not-open-sourced product somewhere? How would you know? How could you possibly know?

A quick Google search on "contributor license agreement" shows 1.6 million hits. Obviously, a great deal of Open Source maintainers and organizations do care, and CLA's are obviously the answer to this legal matter.
In the end, I think the majority of those who've followed the SCO vs Linux court case agree that it is exclusively a matter of halting the progress of the rapidly evolving Open Source world. Microsoft, the father of FUD, assuredly caught onto the dick-grip SCO had on Linux in particular and Open Source in general, and decided to sponsor SCO by handing $12 million dollars to SCO, "to purchase UNIX-type licenses so Microsoft customers can run UNIX-type applications" (this was in the year of 2005, and was reported by Business Week). In the end, though, did SCO win? Have they hampered the development of Open Source software?

In my opinion, yeah. They have won. They have won a fraction of what they aimed for, but yes, I believe SCO got if not the whole cake, they got a taste of it. But I also believe that what they won, the Open Source movement will ultimately benefit from. Ultimately. Eventually.
From personal experience, I know what a CLA can do to the quantity of contributors. A lot of people feel that they want to help a project out, but when they're handed a big, scary paper which may be interpreted as giving up the rights to something you give away for free, they hesitate. And rightly so. Everyone should hesitate when prompted to sign legally binding documents; everyone should read the fine print and ensure they know what they're getting into. But this, naturally, proffers a wholly different stage than the good old "Wanna help out? Just chuck yer code at me and I'll eyeball it and if it looks good I'll plop it into the svn tree, mate!" type of development.

In the long run, though, Linux and Open Source have been children until now, and it's time to grow up and face the big crowd, and the big crowd usually wields lawyers like children wield wooden swords, and the difference is that more than a few bruises and tears are at stake. That SCO will ultimately lose to Linux I have no doubts of. And in a way, I am grateful that the world will get to see Linux prevail in court over the devil, and I believe companies worldwide will see this as a trigger to examine Open Source alternatives closer, au contraire to the belief that companies will shy away from it, due to its "run-ins with the law."

Kalle Alm is an open source developer located in Sweden, currently working on SynchroEdit for Alacrity Management Corporation, since about a year ago. He has been using Open Source software (linux in particular) for nearly a decade. He's blogging about this and a variety of other things at http://kallewoof.com/