As I said before it BBM will work over Wi-Fi however eventually RIM will realise that your BIS service has expired and stop you from connecting altogether. Whenever I travel abroad I turn off mobile network hence I basically am not roaming as far as my carrier and RIM is concerned, however I still check email browse and use BBM over Wi-Fi.
If you have never subscribed to BIS meaning its a new phone, you will not be able to use BBM over Wi-fi or even browse as you will not have the correct service books installed and will not be registered with RIM.
Blackberries have a preference for Wi-Fi over EDGE or 3G because the data connection is faster. If you notice the Youtube application will actually work over Wi-Fi and many data centric applications require Wi-Fi to work. e.g. Online Radio. BBM also works faster over Wi-Fi as BBM messages are delivered instantaneously.
Stop trying to beat the system and not pay for BIS. The Bold devices you have mentioned sound as if they are not subscribed and hence will NOT WORK. Give up.
For those that do not believe that you can use BBM over Wi-Fi alone, just turn off mobile network and leave Wi-Fi checked and you will notice that messages still come through. This will only work for users with an active subscription or those whose subscription has expired and have not been deactivated as yet.