Of course it is unlimited, we just don't know what. Sort of like the ads that say "100% pure"
If you are on a BES, there are no charges, if you are on a BIS then you get to play the game.
If you look at your bill/features online, you need to look for anything that says "pay per use" probably Internet Express in some abbreviated form, but maybe something else too, not Text Messaging. Cingular seperates the WAP APN access verses other web access and will charge you for it. Calling C/S you have about a 50% chance of them tell you it is included or not. You can search and find many threads here about the overages, the lastest is about a $1200+ bill.
That said, I had no charges for 4 months, then switched BBs and have had charges ever since. I call each month and they remove the charges without even questioning it. I am assuming one month they will not remove the charges (mine average about $40) and I will cancel my service and move on.