I'm in the same boat as you as I'm looking to change my Treo for a Pearl, although unlike you I want to have voice and e-mail in one device. After trawling the networks' sites it looks like there is often a dearth of information and the plans available aren't that attractive. For example:
Vodafone: £15 p/m incl. 6MB e-mail data
Orange: £17 p/m incl. 6MB data (and 25 texts) or e-mail & voice at £14 p/m + the voice tariff
Orange just list a "data allowance". Wishful thinking would be that this is for just the extra downloads of attachments etc., but nothing is specified so I assume this is to cover everything.
I know that BB has much tighter compression compared to how I use IMAP on my Treo at the moment, but I often use more than 6MB at the moment and that's with just using e-mail perhaps every three days on average for an hour or so at a time!!! Therefore I would think that the 6MB limit would defeat the object of BlackBerry and I'd have to have it turned off most of the time for fear of receiving a massive bill for going over the limit!
I would be really grateful if someone in the UK who does use BlackBerry (in particular on Orange) with BIS could tell us what tariff they're on and how they manage
Any advice much appreciated!