I think we need to separate between two issues. I'm sure that in large organizations there is a formal process for approving free open source software, because of issues other than cost, such as liability and other corporate policies. But on the other hand, in many cases open source software is "creeping in", because there is no cost involved so the developer/architect doesn't need to go to anyone to get the money.
Now, it could be that developers just start using the open source stuff for prototypes, testing and such, and it comes to a point -- for good reason -- that the use of this software is a fact of life, and it is purchased through a more formal process.
I'll give you a small example from our own -- not very large -- company. Take a look at this case study
Atalassian published on how GigaSpaces uses Confluence for our documentation. Our tech writer, just downloaded the software, customized it and came to us (meaning the decision-makers) with something that was a no-brainer.
He couldn't have done all of that with a traditional closed-source, paid-for product. Maybe he wouldn't have bothered making a list of vendors, asking for approvals to get proposals, talking to sales guys, etc. He was empowered by Atlassian's open model. Although eventually, the purchase went through a more formal approval process.
GigaSpaces - The Scale-Out Application Platform