Actually, I have a requirement just like Marioletto.
The reason I bought the iPhone was specifically so that I could share it with my 3-year old son to keep him occupied during busy times (driving, shopping, etc). He knows how to use the apps brilliantly, and itunes have great educational apps for kids! I use the Restrictions section to prevent him from unnecessarily using 3G/Edge (and getting Data charged for it!). But I can't prevent him from making accidental calls (luckily the few times he's accidentally made a call was to my husband, and not a client!)
So if anyone is still following this and has perhaps found a solution, please do share. Thanks!