yeah everyone's pretty right abou the weather. About now is likely to be just perfect up there - but i know people who go there regularly in november and even january but you jsut have to pick you time.
You'll also find that there's places you can't climb at what ever time you go there. if you go in december don't expect to climb anythign north facing (this means the great wall) but the Ramparts will be in shade by 11ish, and i always think that they tend to work like a giant heat sink as its generally nice an cool, and a lot of moonarie's great lines are in this area. as anywhere though, if you get a hot spell it won't be very nice.
camping up at top camp is as it has always been. You've always needed to keep a low profile up there because its in a national park. There was an incident about a year ago when some guys from canberra were benighted on the great wall and a local policeofficer got out the SES unnecessarily and some ugliness ensued. They ended up with fines for bing in the national park without a permit - but this was pretty much an abberration.
i'd expect there to be a fair bit of water in the tank as its been fairly wet, but i don't know for sure as i havn't been there since about May. its generally good practice to carry up a litre or two of drinking water each day to minimise tank use - but if you're camping up there you'll probably rely on it a lot more.
The walk isn't as long as ppl suggest - it just feels like it. you should always leave your gear up there after the first day and you'll be bounding up and down the track - especially if you leave pretty early when its not too hot.
I'm going to be trying to get up there a few times towards the end of the year - i finish uni on 7 november (forever - finally) so drop me an email when you're going up as me or some of the other locals are likely to be around and can point you in the right direction.