The recent report by Gartner, the State of Open Source 2008 (http://www.gartner.com/; report G00156659), as summarized on their site provides some very interesting conclusions:
1. By 2013, a majority of Linux deployments will have no real software TCO advantage over other operating systems.
2. By 2012, 90% of enterprises will use open source either direct or embedded.
3. By 2011, open source will dominate software infrastructure for cloud-based providers.
4. By 2012, software as a service (SaaS) will eclipse open source as the preferred enterprise IT cost cutting method.
I agree with Gartner that open source will continue to penetrate more companies, but I think that it will occur much more rapidly than suggested by Gartner. And they are absolutely correct that use of open source is "elusive". We find that virtually all of our clients use open source even if they are not aware of it. Gartner captures the reality of open source use in their statement that: "Users who reject open source for technical, legal or business reasons might find themselves unintentionally using open source despite their opposition."
I don't agree with their conclusion about Linux and SaaS. I agree with the skepticism expressed by Mark Taylor http://news.zdnet.co.uk/software/0,1000000121,39379900,00.htm. My experience is that the use of Linux continues to grow rapidly and it is likely to take an even more important role in mobile devices. The statement about SaaS confuses a business model with a method of developing software. Many open source companies use SaaS as a distribution model and it does not make them less "open source." http://lawandlifesiliconvalley.blogspot.com/2008/03/open-source-overview-from-osbc.html
The report once again emphasizes how open source is becoming part of the mainstream. A decade can make a big difference: "Microsoft: Resistance is Futile" http://www.news.com/2009-1023-229218.html.