I was told the other day that BBM is free north america wide. However, BBM works through your data plan, so you must have data turned on, which means that your phone will download email, etc. which WILL incur roaming charges. I suppose you could turn off your email by removing the email settings, and then be sure not to use your phone for browsing. (Browsing is probably the biggest data roaming cost)
In practicality, what we do is just use BBM, let it download email (but never retrieve attachments), and never browse, and it costs us less than $10 additional per trip for data roaming. The last trip I believe my husband either used his browser and/or sent email and the data roaming was $40, so if you think you may want to use these features you should get the $10 US plan for peace of mind.