When I fly on business, I typically have _two_ laptops with me, a Titanium PowerBook G4 with Mac OS X and a HP OmniBook with Windows 2000. Only once was I forced to turn them on - most of the time, I take them out of the laptop bag, send them both through each in their own little plastic thingy, and then my bag. After screening, everything goes back in the bag.
At DEN, they even have signs about 20' feet after you're out of the security checkpoint (at least on the bridge to A) that say "Got Laptop?"... One time, I didn't -- it was a very good thing that I saw the sign.
Since 9/11, I've taken 20 trips through security with both laptops, and never has my bag been taken from me to check... whoever told you that you can't take a laptop bag onboard is mistaken.