In the lab we're chatting about what effect a PhD has on your career and your earning potential. This article is slightly old (2001) but it has some solid figures which are interesting:
Seems that a PhD in an electrical-engineering discipline (the closest match to ours) could raise your salary by around 8 or 9 percent.
Of course the economic car-crash puts a lot of things in question. But I'm glad at least that a PhD doesn't on average push your salary down, which some people say (and maybe it's true for some disciplines).