I had to move from Windows platform to Linux recently for my project development. I had OpenCV2.1 installed and configured on Windows which was very simple. However, installing OpenCV in Ubuntu wasn't. I had a look at several method before finding one that worked.
OpenCV's previous versions were different and needed a different install method. Version 2.1 makes this slightly easier but still tricky to follow. WillowGarage's install pages didn't work for me and I couldn't use OpenCV. but I found a blog post which explain it clearly and step-by-step. Follow this blog to install OpenCV2.1 on Ubuntu.