The Occupational Outlook Handbook, 2010-11 Edition published by US Department of Labor includes section for interpreters and translators, from which you can find many interesting data or points. For example,

* (US) Interpreters and translators held about 50,900 jobs in 2008. (They noted the data might be heavily underestimated. I think they're right.)
* About 26 percent of interpreters and translators are self-employed.
* Employment of interpreters and translators is projected to increase 22 percent over the 2008–18 decade.

The full article can be found at http://www.bls.gov/oco/ocos175.htm

While the data are based on USA and sounds not so reliable, I think it's a good news to all: "Interpreters and translators can expect much faster than average employment growth." Cheers!