In the pre-9/11 era I was once asked to turn on a laptop. Fortunately they didn't want to see much--the battery pack was in terrible shape and couldn't keep it running long enough for the desktop to appear. (I knew about the problem but it wasn't a big deal as I was always using it plugged in anyway.)
Since 9/11 I haven't flown in the US without a laptop and I've never been asked to turn it on.
Likewise, pre-9/11 I once had a bit of trouble with a SLR. The screener apparently didn't understand SLRs and thought it was fake because she couldn't see anything through it. Never mind that that's all you normally get when the lens isn't mounted.
Post 9/11 I have generally carried a DSLR and the only time it's drawn any interest was a domestic flight in China--and that turned out to be due to the batteries for the flash, not the camera at all.