I strongly believe that logic plays a big part in telling lies. Logic allows you to convince people that your way is the right way, by carefully pointing out all the things they need to hear and conveniently forgetting the ones they don't. The only problem with Diplomacy comes in the fact that everyone is naturally skeptical, and a lot of times people won't believe you just for the sake of not believing you (the whole, "This is too good to be true, something must be wrong" mentality).
Though, in other board games, I've often been able to convince people to do some pretty silly things just because I presented a case that seemed so logical to them that they figured they couldn't lose. (Mind you, I don't have a degree in logic, but in the one logic course I took in college, I cruised through to an easy 4.0.)
I dunno, logic is just a hard thing to refuse. If you try to convince people to do things with just your winning smile and some friendly gestures, they might be on the fence. But when you lay out an argument that seemingly has no holes, it's hard for them to disagree.
Unless, of course, they have a greater grasp of logic than you do, and they can see those things you conveniently left out. =P