Rob
I want you to get confirmation from o2 about your charges as they scrapped additional wap charges in February 2008 as I explained before.
Here is a link from the o2 website regarding BES packages (if I remember correctly you said your were on BES) it clearly states that the only extra charge is for mobile web. Mobile web is a different gateway from 02 wap and this is only used by Opera Mini and only if the mobile web tcp/apn settings are entered onto the device. If these apn settings are not entered then there is no danger of accidentally going to the mobile web. I have never had any mobile web charges.
BlackBerry from O2 - Email and Internet on the go
BBmessenger is a RIM application and therefore uses Data from your package. Practically all 3rd party applications will use the blackberry.net gateway.
It may well be that when you got your plan o2 sent you an email regarding the terms & conditions of your plan, they sent this to me too and I discovered that it was old out of date literature.
Please take time to find out as it will put your mind at rest about data charges. Make sure you talk to someone who knows about Blackberry Tariffs as customer servives in the UK regarding Blackberries is not the best to put it politely.
Do you have an online account? you should be able to log in and look at your bill and there should be a page there showing you what you have used so far. Go to recent charges and select internet & browsing. It will itemize if you have used blackberry.net or o2wap and if there are any charges for it.