A few folks have mentioned server lag on different threads, and I wanted to mention that there could also be some ISP-side lag to be considered (at least in some of the cases). Our ISP is supposed to be providing cable service (I'm not convinced), but I've had plenty of moments of ineffectual mouse-clicking, weird teleporting, rubber-banding, most of which were accompanied by my partner cursing in the other room while trying to do work online.
It seems that our ISP intermittently pulls the plug when it feels that too much bandwidth is being used (and yes, we do pay for both cable lines).
I had a couple of lag moment also but it was mostly due to my MSN being online.
People must not forget that every other programs running on the computer while playing can also be the source of lag.
As for myself, each time someone would send me a message in MSN i was lagging in game. the lag was even syncronized with the MSN window flashing.
I have noticed that the servers usually lag right before they say a new build is coming out, etc. I think it's due to their streaming updates thing. It's nowhere near as bad as WoW tho...
I always lag when the little streaming lightning bolt pops up, but the absolute worst came during a tomb battle, and everyone on the team was complaining about it, so I doubt it was coincidence =\