you can turn on international email plan but that just covers exactly what it says...just email. if you use data in any form as in BBM or web they will charge you. not sure how much tho. last time i checked they don't offer a international data plan anymore. they just made a plan for email and jerked us travelers on the other data with no plan at all. so your are always roaming on data. now that's DIRTY...
If you're just going to Italy, you might want to consider unlocking your phone and getting a pre-paid TIM or Wind SIM card while you're there for your use. BBM PIN remains the same, people can still reach you through messenger text or email, though your phone number will change. MUCH cheaper than TMo's $15/mb data charge.