Messed up system you have there. In Australia, all networks are GSM, and compatible with each other (assuming your handset supports the frequency of the network you want, which USUALLY isn't a problem). Because of this, it's simply a matter of changing SIMs if you want to change networks. That's the way it SHOULD be. Sometimes a handset will be locked to a network, but the carrier well unlock it, generally charging a fee on the order of $20 to do so.
In addition, our plans seem to be cheaper (probably BECAUSE people can switch networks so easily). Phones do not carry an upfront charge, or activation fee when you sign up for a two year contract. That being said, more expensive phones tend to require a higher plan (eg, a galaxy ace can be had on a $15 plan, whereas a galaxy s2 would be more like a $35 plan). The main difference being that those plans include both calls/text AND data for the one fee.
The flipside is however, that our ETF's are equal to the remainder of the contract, making what you suggest impractical.
I see that those four networks have figured out that the key to royally screwing you guys is making it very difficult for customers to move between providers.