A recent thread on the Apache HTTPD development mailing list reminded me of something funny. Orthodox Server Programmerism states that events are better than threads. Funny thing is that at the same time this meme has finally broken into the mainstream (the last several years) it has become largely irrelevant. Even better, it is only going to become more and more irrelevant as time passes.
OS kernels, even ones that love events and hate threads now do threads very efficiently. On top of that, many-processor, multi-core computers are the norm (heck, my laptop is dual-core) and this trend is going to increase very quickly.
I suspect events vs. threads is going to go the way of Assembly vs. C (or C vs. Java (or Java vs. The P(+R) gang)). Sure you can theoretically optimize the former better, but the theoretically will be operative in the vast majority of cases. Heck, I hope thread schedulers use events when a thread blocks on IO. Presuming they do, the penalty for a thread vs an event listener should be the thread stack and restoring registers. The thread stack is a specific solution to storing the context for handling the event, and I don't know close to enough to dive any deeper :-)
Heck, anyone designing a new system for embarassingly concurrent stuff today would probably be better off solving this in the compiler and then exposing linear programming to the programmer via explicit happens-before semantics rather than a thread model.