This is incorrect. Redis can swap out to virtual memory when it runs out of RAM. Although, it should be leveraged with some care (http://news.ycombinator.com/item?id=2606096)
Thank you for the link. I actually do remember reading that, and I guess I completely forgot it was even possible. After reading about it, I told myself that redis will be in-memory only for the solution I was using it for. I'd definitely be curious to see some real world performance numbers for redis using the disk VM before considering it an option.