Hi,
I have recently spoken to a lot of people who are a bit lost when trying to find out info about wireless broadband.
So I thought I will put together a list of things & sites which will be helpful in the process.

The main thing to remember is that as long as you are in a 3G zone which at this stage covers 65% of the Australian population (this is what I gather from the Optus website & higher coverage from Telstra) you will be able to achieve practical speeds between 500 kbps to 1500 kbps (although the companies claim the max speed possible is 3 mbps). However when you go outside a 3G zone you will be on the GSM network which means you will only achieve speeds between 20 to 50 Kbps.

So before you go out and buy a 3G connection please make sure you are in a 3G coverage zone - I have provided two links for the wireless coverage checks for Soul/Optus and Three:

http://www2.optus.com.au (Optus/Soul)
http://www.three.com.au/cs/ContentServer?pagename=Three%2FPage%2FIFramePageTemplate&cid=1185943280265&c=Page&p=&homeId= (Three)

Another important factor to keep in mind is that some companies diffrentiate between the charges for the 3G zone vs the GSM zone. E.g. from what I can understand Hutchison's 3 Network charges users roaming charges when they go outside Hutchison's 3 zone and this can hit your wallet big time as the roaming charges are very expensive ($1.65 per MB above the roaming allowance)

Telstra is supposed to have the best coverage amongst all the players & from what I can gather they cover more than 95% of the Australian population. However they are much more expensive than Optus, Three and Vodafone. I would recomment Telstra wireless to people who desparately need broadband internet at whatever cost.
http://www.beeneverywhere.com.au/html/pricing.htm (Telstra pricing)

One of the best networks I feel is the Optus network as long as you are in a 3G coverage zone because they do not charge you excess fees in roaming charges for wandering between 3G and GSM zones and further their modem/software automatically selects the best network which is available.

The Optus wireless connection is sold via many resellers and one of the best plans around for people on a budget is the $19.95 for 1GB download plan from Soul and at the time of writing they were giving away a USB wireless modem for Free on a 24m contract. Please see www.WirelessInternet.net.au for more details.

However for people who want to do more with the Internet, check out Vodafone's wireless internet option $39 for 5GB download
http://www.vodafone.com.au/Personal/PricingPlans/VodafoneMobileBroadband/index.htm