We at Black Duck noticed it too. So far, looks like the decline is largely because, lately Ohloh - a) hasn't kept up with discovering new projects. b) hasn't paid needed attention to keeping existing projects up to date.<p>We'll provide more details once transition is over (we just acquired Ohloh from Geeknet).
I suspect it has more to do with the mechanism by which commit volume is measured than the actual commits themselves.<p>It's possible people are just moving away from verbose languages like C and Java and thus just needing to write less code, but I'm not sure I have enough faith in the human race to believe it.
Hum. The App Store opened in mid 2008 and the Android Market in late 2008. There might be a link.<p>Also, there is a growing quantity of proprietary Web Apps which might have helped shift interest of many users and programmers along.<p>I'm not all that surprised as it has been one of my concern lately. The Open Dream might come to an end with the make-it-all-online meme.
What about the possibility of the emergence of distributed version control? Perhaps this is causing less actual commits to the actual project, but not necessarily less code being written.
HTML, JS, CSS, PHP, and Python all have steady increases, and ruby is mostly stable. It could just be that focus is more on the web (HTML in particular seems to have taken a biggish jump mid-2009 - related to HTML5?)<p><a href="http://www.ohloh.net/languages/compare?measure=commits&percent=&l0=html&l1=javascript&l2=php&l3=python&l4=ruby&l5=xml&l6=css&l7=-1&commit=Update" rel="nofollow">http://www.ohloh.net/languages/compare?measure=commits&p...</a>