It's important to note that "open source" is a very broad term that covers a wide variety of business models. There's software that is completely free, including the support (or lack thereof). There are some incredibly awesome programs that fall under this category with great followings and very active forums. But even in the best case scenarios, there is still a risk of encountering an obscure bug that no one else has found, and in those cases, you have no idea when you're going to get a fix. There's no SLA, since you're not paying for a service. No matter how mission critical your problem is, no one is realistically on the hook for getting you a solution - ever. That's not saying a fix won't eventually come out, but there is no guarantee when it comes to completely free software and support.
Of course, some of you guys are like, "just look at the source code, stupid!" Honestly, maybe I'm just not as great of a programmer as some of you, but the code for a lot of very complex programs is...ummm...complex. As in, unless you were actively participating in developing that software, you probably have no f-ing clue what you're doing. You may introduce more problems than you "fix." It takes weeks/months to completely ramp up new developers for some of our more complicated projects, and that's usually with guidance from the original programmers. Now, during a production emergency, you're hoping that one of your software devs (not sysadmins, since there's a difference) can take the time to go digging through the source code, hoping/praying that there's SOME comments/documentation...
Some open source companies, as someone mentioned above, give away the software, but sell support. Red Hat (full disclosure: my company is a subsidiary of Red Hat) works on a variation of that model. Yes, they have software that is free, such as Fedora or JBoss. Those "community" versions have 0 support, although fixes and new features are constantly being released. Rapid development occurs in the community/free releases, but no guarantee is made on stability or even the functionality. Red Hat then takes promising releases and after some GA and CR iterations, produce an enterprise version of their software. People pay for the enterprise license, and with that, finally, comes support. This mitigates some of the risk that corporations traditionally see with open source software. You'll hopefully get a lot less "what nightly build are we patching from" or "I hope this version of x works with that version of y."
Overall, I think proprietary and open source software purchasing decisions should be made based on needs, not on ideology.