The European Commission published an interesting study on the impact of Free and Open Source Software on the European economy (PDF, 287 pages). Based on an empirical study of the European IT market and forecasting techniques, the following is one of many interesting conclusions:
Proprietary packaged software firms account for well below 10% of employment of software developers in the US, and "IT user" firms account for over 70% of software developers employed with a similar salary (and thus skill) level. This suggests a relatively low potential for cannibalisation of proprietary software jobs by FLOSS, and suggests a relatively high potential for software developer jobs to become increasingly FLOSS-related. FLOSS and proprietary software show a ratio of 30:70 (overlapping) in recent job postings indicating significant demand for FLOSS-related skills.
I can't speak for other projects but Drupal is in high demand, and there is a shortage of talented Drupal developers and consultants on the market.
Either way, make sure you can put "solid understanding of open source software development" on your resume. It increases your market value. And you'll have more fun too.
— Dries Buytaert