I was in the UK with my Vodacom South Africa Blackberry Bold 9700 on SMS roaming. The setup worked great as I could SMS when I was out and about and make use of full BIS functionality when I was in range of a WiFi network. I was receiving push email, using Twitter for Blackberry, sending BBM and making use of all my applications as I usually would back in South Africa. BIS over WiFi really works great on the Blackberry Bold.
However, I made the mistake of inserting a Vodafone SIM card into my BlackBerry to make a voice call. I didn’t realize that Vodafone would push its Blackberry service book settings onto my BB automatically. So at that point my BB was reduced to plain old WiFi hot-spot browsing. I lost push email, Twitter, BBM etc. and I couldn’t find a way to restore my Vodacom BIS settings manually. I had to make do with using the BB browser (and Opera Mini) for all my web needs. Of course I could have just signed up for a BB data plan on the Vodafone SIM but I figured I would learn my lesson and just manage without BIS.
Within 10 minutes of returning to South Africa (and deactivating SMS roaming), I received push email on my BlackBerry again – Vodacom had restored my BIS settings. I don’t know if there is a way to prevent your BB from downloading provider service book settings automatically, but do be aware that your BIS can be messed up if/when you use an alternative provider’s SIM. This can be particularly troublesome when you are on SMS roaming, as you won't be able to get your native provider's BIS settings back until you return to their local network.