I can order a copy of my drivers license and change the address to "1600 Pennsylvania Avenue, Washington, DC" and they'll ship it to me quick as can be. If you happen to find somebody who looks a lot like you (or you can modify your face to look like them) you can just get their social or drivers license # and order a copy to a different mailing address. The original owner is never notified.
If you want to just make yourself a brand new ID under someone else's identity you can get a new driver's license under someone else's name. Bring two forms of ID (a birth certificate which you can order with a SSN or a faked certificate of US citizenship along with a faked insurance card), a faked telephone or utility bill, and a proof of social security number (a faked payroll statement and a faked insurance or IRS statement with name and SSN). Then make an appointment at the DMV and get a fully valid new government ID with your picture and someone else's name.
That's something a terrorist could do without much trouble once they had someone to target. You don't even need Photoshop to fake those documents; just a Kinkos, MS Word, Paint and some scissors. Just get the guy's name and SSN (which thanks to LulzSec there are plenty of leaked to the internets) and go to town. Should take about a week, maybe two if the fake birth certificate takes a while to ship. You can also do without the birth certificate if you just order a copy of the person's drivers license.
So yeah. The fact that the TSA doesn't check your ID past glancing at it and putting a mark on your ticket isn't that scary to me, since anyone who really wants to can get a valid ID which will pass all checks in a couple weeks.