Right after I graduated with my BSCS in 2002, I went into the Army for four years (which is how I got my clearance). The only employers willing to even look at me ("if you love programming so much, why haven't you been doing it for the past four years?") were defense companies, and only because I already had a clearance. :)

We sometimes hire fresh grads without a clearance, but experienced hires are generally expected to already have them. It's not cheap (and definitely not fast) to get someone a clearance. Though, some companies/contracts/projects are probably more flexible on this, especially if they have something unclassified you can work on while they wait for your clearance to come through.