Hey SuperSteve,
Yachris is right on the mark..the only other possibilities for "hidden" charges would involve international toll charges if you make a call to an international number or if you were close enough to the border, Canada or Mexico and you handset attached to a tower outside of the Nextel/Boost footprint....rare, but I have seen it happen.
Also please be aware of getting drawn into "subscription" applications via your Wireless web...chat rooms, instant messenger, joke of the day type crap is a drain on your funds and is often tough to get these third party leeches to stop.Let us know if there is anything else we can help with !
Jess