Customer in Sydney Australia should use which linode farm?

Geographically-speaking, you're probably looking at Fremont, although you can try all five here:

http://www.linode.com/speedtest/

Given Australia's high cost of Internet connectivity and disadvantageous regulatory climate, I'd be surprised to see a datacenter there in the future. Then again, if you dig back through the forums, I said the same thing about Europe.

Australia is so far from everything, though, and I believe some providers bill national and international traffic differently, so there's a bigger advantage to having a location in Australia to serve the local market.