Is Encore CS5 any faster than CS3 at rendering and encoding? I'm on an 8 core mac and iCS3 is only using (typically) 1 or 2 cores worth of CPU time (typically 100%-200%).

If CS5 any more multi-core aware?

CS3 seems to render menus sequentially while other cores could be encoding other menus in parallel.

The Blu-ray I'm currently encoding has taken 3 hours to render the menus alone, and that's not including the footage transcode time!

Other than multi-page and popup menus, did CS4/CS5 add anything else over CS3?

I can't really answer your question, but CS3 predates the 8-core-era, so it doesn't surprise me CS3 doesn't 'see' more than 2 cores.
CS5 is created in the 8-core-era, so it would make sense that it uses all the cores. (But this is where I don't know whether that's the case.)

Encore is still 32 bit, because parts of the engine were licensed from Sonic and they have not yet upgraded to 64 bit support. So the answer is plain and simple: NO. No improvements in multi-threading, no improvements in memory use, no improvements in speed.

Thanks Harm. Not what I wanted to hear ;)

In terms of extra features, is it just pop-up and multi-page BD menus?

If you perform your bit calculations early while in premiere, then it will take care of the rendering to the m2v file, and this will not need to be transcoded again (recalculated to be precise) and it will save you a ton of time in Encore. The manual explains this clearly.

It's only when you import a 'master' into Encore and then figure out how much data you need that things start getting complicated.