Hello fellow Aussie
You should be able to use your iphone as a wifi hotspot for your laptop, so if you get a SIM with a decent data plan, you shouldn't have to get a separate dongle.
Also you may want to check out this thread in the Australia forum
http://www.disboards.com/showthread.php?t=2925094
I haven't actually read all of it since it's not of specific interest to me, but I assume that it's more geared towards the US. Still, it could be worth checking out.
Personally I always just use free wifi when I'm away - there are heaps of hotspots around now, and there are quite a lot of hotels that offer free wifi too if it's important for you to be able to use it in your room. All the hotels we're staying at in Europe have free wifi.
For our phones (we generally only take 1), we just get a travel SIM and use it sparingly. We usually find we don't really need to use it, but on the occasions where you need to contact home, travel agents, or insurance companies, it does come in handy. We like the travel SIM because you can use it in a lot of different countries. On the few occasions that we split up, we just do it the old-fashioned way and make a time and a place to meet up again
