I'm sure it varies upon airport and airlines, but we just came back from McCarran (Vegas) on Aloha Airlines (a local airline here in Hawaii), and we noticed that they hand checked carryons for those who didn't check in any luggage and just packed everything in a rolly-bag. We carried on backpacks and were not searched.
We only carried fanny packs and a camera bag to the parks and didn't have any problems.