This is another one of those tasks where I thought it should be simple. That should be a red flag right there. I ended up spending way too much time on this, and as it turns out it is really only a matter of syntax. I ran across a lot of incorrect information out there, so I figured I should post this and help make the links to the right content easier to find.
I was actually building my own cmdLet for PowerShell in C#, and decided I should deploy it using PowerShell, which seemed appropriate. For me, the basic stumbling block for getting this working was quotes. I had multiple paths in my script, all of which had spaces in them, which necessitated the use of nested quotes. This was quite hard to figure out, but Philippe had the right information. He also pointed to this article, which had a thorough explanation of how to call PowerShell from the command line, which is essentially what you are doing inside a post-build event.
Since my cmdLet was really for SharePoint, I later figured out how to add it using the SharePoint 2010 project type in Visual Studio and some declarative XML, making all the previous pain a moot point.