While it is true that Windows still dominates in the office space, there is a sea change afoot in job postings. Most are now calling for Linux experience. From last year to this year, the trend is significant, whereas, in previous years, the trend was slowly creeping up.
If you're still in school, and you want to work with computers, plan on learning Linux. Why? Linux is the platform of choice for Google, eBay, Amazon and just about every big player on the internet. But it's showing up more and more as businesses move away from Windows.
Many businesses are not planning a migration to Windows 8 anytime soon. I can see a day where Linux clients that present a web browser for work will be the dominant platform. I don't think that day is very far off, either.
The Linux Professional Institute offers a vendor neutral certification system to test your skills on Linux. They also offer free training materials that you can find here.
From web programming (java programming is red hot right now) to system administration, if you're going to work with computers, you will need Linux certification to get your foot in the door of just about any IT shop.