I currently live in DC, and the possibility to get a security clearance is opening up to me. I still have high hopes of moving to Silicon Valley once I'm done with my PhD, but if I stick around for an extra few months and submit to questioning, a clearance could be had. Its value in the DC area is somewhat obvious (and huge, actually), but how valuable is it elsewhere, especially tech-heavy places like Silicon Valley, SF, Austin, etc.?
I can't imagine it would be particularly valuable outside of work in defense. If you want to work in defense, then it's valuable because it opens those (admittedly well paying) avenues to you. Otherwise, I can't imagine what it would bring you.
The ability to work for private security firms that are located outside of DC, but handle large DOD and SD contracts, could be a good in to the security IT click.