I think it mostly depends who you fly with... in Aug I flew from UT to Alaska (3 flights) on Alaska/Horizon Air, and nobody had any problems with my wearing my 7mo through security, in the plane, etc. Just last month I flew with him again (now 11mo) again on Alaska, and again, no issues... I had heard that they would want me to take off baby and sling to go through security, so I had done that (metal rings on my sling--sent it through the x-ray machine in the bin along with coats and shoes etc). But in the plane, yeah, I don't have the money to buy a ticket for an infant that can go on my lap... and I figure if he's on my lap, he's safer in the sling than out of it! I didn't get harrassed by ANY flight attendants or anybody. One stewardess saw me nursing as we were taking off (we were back row on a little plane, so she was actually sitting almost exactly next to me) and she commented that lots of moms nurse during takeoff/landing, because it's a good way to help the baby's ears adjust to the altitude changes...I hadn't thought about that, but since then I always try tog et him to nurse when we're taking off or landing. :-)
So, yeah, I've heard horror stories about having to pass the baby through security separately from you (ie, can't carry them through, have to hand him/her through to another person) and stuff like that. I have gone through security at three airports now though--Juneau is super easy going, Seattle is super paranoid, and Salt Lake is somewhere in the middle...I went into it figuring I would just do whatever they asked, but I didnt have any troubles at all.