I carry a Spyderco Delica, a folding pocketknife which has about a 3" blade, but is serrated, has one hand opening thanx to the thumbhole, and basically looks mean as hell.
I've travelled quite a bit the last few years, and I carry my knife with me out of habit, since I wear it everyday. When I don't wear it, I find myself slapping my right hip a good bit, which is a kind of nervous tic the habitual Spyderco user finds himself developing.
The inevitable happens, which is I forget I'm wearing it, and walk right onto the aircraft. When I land, I realize that I made it through, most likely due to the low ferrous content of the stainless steel blade and plastic handle. So I wear through on the return flight, without a hitch.
This is until I encounter the brand spanking new metal detector at the Columbus, Ohio airport. I set it off like a pinball machine, where the very zelous and motivated security personnel quickly locate it with a wand. I have to leave it at the customer service desk. Luckily, I was only picking someone up, or I'd have been S.O.L..
Living in LA now, where I would describe the equipment as somewhat sophisticated and the people as wholly unmotivated, I unclip and tuck it into the cellphone pocket of my carryon bag, so that it's sandwiched between the LIon battery of my phone and laptop. Both batts are radio-opaque, so this works out. I guess this might be foiled by a 3-d cat imaging xray machines like the one at CHI, but otherwise I'm good-to-go.
UPDATE: 9.13.2001
No more knives on planes. Not mine, not anyone's. If I see you with a knife on a plane, I'm going for your windpipe. I don't want to possibly provide a knife to someone on board - I don't want to figure out how to get through the new security, and if I do, I'm sure not going to post it on the internet. I'm serious - if I see you on a plane with me, and you're carrying a knife, I will seek to disable you first and ask questions later. Maybe this is chest thumping, locking the gate after the horse has run, but it's my position now. I leave the above as a demonstration of how we got into this mess in the first place.