Brief history (of the ones I've attended - I missed out on the first few ) - in 2009, it was held in meeting rooms on level 12 of the Victoria University of Technology building. I can't remember exactly, but I think the public expo had around 2-3,000 people come through.
In 2010, still at VUT, we had around 4-5,000 people come through, with queues stretching some way around the block (since the traffic flow was limited by the speed and capacity of the lifts). This brought us to the attention of the Melbourne City Council, and after much hard work from the committee, Melbourne Town Hall was arranged as the venue for the next year.
In 2011, the event went well, but word had got around - we managed to get over 7,000 people through, and turned many (hundreds ?) of people away. Many more probably gave up upon seeing the queue stretching completely around the block, with an estimated 2 hours to wait before being able to get in. The hall was packed to its limit all day, and the committee were rather stressed by the end of it.
So, this year. Still in the Town Hall, but with two major changes - tickets were to be sold in advance for specific session times, and there would be two days of public display.
There were some trials along the way (e.g. when the booking site went down for around 24 hours due to a data centre move only a week before the event !), but all 10,000 tickets were sold by Thursday morning last week, much to the relief of the committee I imagine.
The public display days ran much more smoothly than last year - the queue for entry to each session was cleared in around 20 minutes, the hall was never over-full, and the lulls between sessions meant there was time for the committee and volunteers to take breaks when required.
Actually, that was another change this time - bringing in a bunch of volunteers for crowd management and general helping out - which again took some of the burden off the committee, and left them some breathing room to deal with any problems which did arise.
The only real issue was getting a bunch of publicity (radio and TV) after being sold out - in some cases, this wasn't mentioned, leading to people turning up over the weekend thinking it would be possible to buy tickets.
Anyway, the weekend was very good, the displays were amazing as always, and I'm sure everyone is planning their builds for next year even now (I know I am !).
You can see some of the creations here, and some of my photos here (more to come, for some reason iPhoto keeps "stalling" in the middle of the Flickr upload - very annoying).
And on to next year, and another move to a larger venue - this time the Royal Exhibition Building, in Carlton. Hopefully the larger capacity and slightly more suitable space should lead to an even better experience for everyone who goes along. Keep an eye on the Brickventures website for updates on the new venue, ticket sales, and registration to the "convention" day for AFOLs, TFOLs, and JFOLs. Time to get building !
 Due to being on the wrong side of the planet :-) 
 Weird, Brickvention has no Wikipedia entry ...
Original post on Dreamwidth - there are comments there.