I'm not sure how it works yet but the newest version of the App Store app on your phone allows you to actually check youself out - no blue shirt needed.
Not sure how they prevent theft yet though, I haven't yet used it.
I used that feature last week and it's quite amazing. Like you, I'd be worried about theft though.
I literally walked into the Apple Store, to the back, grabbed a Bumper off the wall, opened the Apple Store app on my phone, scanned the Bumper UPC, paid with my Apple ID, and walked out of the store. Took about 4 minutes total.
And I definitely felt like I was stealing something.
Not sure how they prevent theft yet though, I haven't yet used it.