Ah but that is just a play on words! Eg the $49 optus cap give you $230 "value" of calls/sms/mms/data at the rates specified for the cap. Once you exceed the $230 value they start billing you at those rates mentioned.
So if you can stick under the capped amount you are fine, the moment you go over, you're paying higher rates than a non cap plan and the bill racks up very quickly.
Depending on what you're like with phones, also look at 12mth contracts, since most don't last a 24mth period given the warranty is half that.
Al, have a look at http://www.phonechoice.com.au for an overall comparison of the plans, don't take it as gospel as I'm not sure how often they update it. But should be a good start anyways