We came up on this factoid in one of our 2003 newsletters:

According to a recent survey (March 2003) by Business 2.0 magazine, the average salary for a technical writer in America is $52,875.00. Just out of curiosity we looked up “today’s salary” at Glassdoor.com – which was updated 27 May – and found the national average to now be $57,000.

By the way, Business 2.0 no longer exists; it was bought by CNNMoney in 2007.