My development machine is Windows7. I do web and systems development in Python, PHP, etc.
All of my development is done on Ubuntu and CentOS VMs running in VirtualBox.
My actual IDE is often Eclipse (or Eclipse based) and I just make an SSH/SFTP connection to each virtualbox and the coding is treated as a "remote project" in Eclipse.
When I retire this Sony Vaio, I'm really thinking I'll get a 15" Macbook. Just for variety. There was a time when I'd never have dreamed of that. I bill a high hourly rate, I cannot be fumbling around because I'm on an unfamiler OS.
But I could switch to a Mac in 60 minutes. Downnload a JVM and VirtualBox for OSX, move over all my VM images, and I'm done. Back to work.
I chose Windows 7 because IMO it's the best OS available for me. I enjoy it. I never have reliability issues. I find I usually go 30, 40 days between restarts. It's nice I have this option: the chip and OS makers have really done a smashing job on visualization technology over the last decade.