On every century or metric, or other, ride that I've done (where drafting is allowed), I've joined pacelines made up of unknown riders. Not only do they always let me in, they, in fact, invite it, waving me in or saying something. Among the club riders, I'll allow them to do their rotations, while sitting in the back, and ask if/when they'd like me to take a turn up front. Among less skilled riders, I assume that we'll all be taking turns at the front. I make an assessment as to their skill/fitness to determine if I'll fit in. Contrary to all the crying here, everyone is gracious, helpful, and we have that much more fun for it. I'm not that uncomfortable zipping along in the mid 20's, with mere inches separting riders and wheels, for miles-among riders I've only just met. That, to me, seems much riskier than the occassional commuter who approaches and may or may not "sit" on my wheel on my 9 mile commute.