You can use your Blackberry browser including the ability to download and have your email sent to your phone using third party push email apps just by installing a "browser via tcp" service book and inputting the relevant tcp settings for your network.My work as a journalist regularly takes me to parts of the developing world where there is no BIS and no network that I can roam on so I use local sim cards in my Bold and use the Shangmail push email app to push a couple of different email accounts.
Using this method you could use an O2 PAYG sim on their £10 tarrif but you would not be able to use some functions that will only work via BIS such as Blackberry Messenger,but your browser would still work and there are a selection of third party push email apps available.I discovered this method a year or so ago and it has always worked very well for me when I have been working in the proverbial back end of beyond.
So the bottom line is that to use your Blackberry browser and have push email,albeit using a third party application you do not need a Blackberry data plan but to use all of the features of your Blackberry you do need a data plan.When I get home to the UK I just pop my Virgin sim back into my 9000,resend my service books if needs be and switch back to BIS again.
I first discovered this method by reading the information
here