So, I will be tethering my N900 to my laptop when I get to Australia and want to know what the difference would be between using JoikuSpot Premium or Bluetooth Dial-Up Networking. (I hope I'm correct in assuming that both programs can be used for tethering.) Is there an advantage of using one over the other?

Well, JoikuSpot allows multiple clients to share the N900's internet connection and creates an ad hoc locale network among all of those clients and the N900 using 192.168.2.x (by default) addresses. This is VERY useful for local file sharing and such.

Bluetooth dial up networking only allows one client to use the N900's internet connection and creates no local network even among the single client and the N900.

Why is USB not an option? Its always my preferred choice as that way you aren't draining the N900s battery when tethered, only your laptops.

The only question then is what uses more, your WiFi chip (which you can then turn off if using USB) or charging the N900 - mostly depends if the N900 is already fully charged I guess.

Although my preferred is probably JoikuSpot with the N900 plugged into USB or the charger. (assuming the full JoikuSpot works, I couldn't get the Beta to work for me)

Well. I will used JoikuSpot Premium, straight forward and easily to use. Tried over USB and Bluetooth Dial-Up Networking doesn't work for me.

Also, BT is slower than WiFi. I think BT is limited to 1-2mbit whereas 3.5G can be faster so your 54mbit link via WiFi can actually make use of it.