I wish I had more statistics to look at this, but at a preliminary glance, it looks like Drupal has accounted for all new PHP job openings in the last twelve months. I just published a little CakePHP, Drupal, and LAMP PHP jobs survey, and while the good news is that the PHP job market appears to have grown well in the last twelve months, the case can be made -- based on these statistics -- that Drupal accounted for almost all of the new PHP job openings.
As you can see in that earlier article, Dice.com reports an increase of 112 general PHP jobs, while they also report an increase of 131 Drupal job openings. Careerbuilder shows an increase of 81 PHP job openings, with a corresponding increase of 63 Drupal jobs.
If you prefer graphs, here's a quick look at my PHP job openings comparison chart from Dice.com:
(Because I use CakePHP, I'm glad to see that the CakePHP job market has also increased, and while the CakePHP job increase percentages are even higher than Drupal's, the numbers are still fairly low.)
Without any more statistics than this, it looks like the growth of Drupal has accounted for almost all new PHP job listings in the last twelve months. While that's pretty cool, I need to add the caveat that I didn't look for Wordpress, Joomla, Ning, and other PHP-type jobs back in 2009, so other than the raw growth in the Drupal job opening numbers, I can't be sure what sort of "churn" was involved in the other LAMP/PHP job listings.
One thing is clear from the Drupal job listings of both Dice.com and Careerbuilder - The Drupal job market has increased by 100% in twelve months, and in a down economy, that's good news.
Drupal jobs update, August, 2011
I just checked again on August 1, 2011, and Dice.com now shows 373 Drupal jobs. CareerBuilder has also increased from 63 Drupal job postings to 211. In other words, Drupal is still growing strong.