I've been using it for over two weeks now and I haven't incurred any data charges. From what I have come to understand, developers have to pay RIM something like $2,000 a year to have their apps use BIS and, from my further understanding, Opera developers have pay such amount to RIM. Contractually, it would mean that , from RIM's perspective (and by extension, other service providers of RIM product BIS) Opera is covered under BIS.
The current Vodacom/ MTN/ Cell C (and probably 8.ta) forward message (you know that one that they spit out as a general answer to everything but answers nothing) names Opera as one of the apps that incurr data charges but that list was made over two years ago when the Opera Mini of that time DID incurr data charges. Several versions later, Opera started using BIS but, I guess, our almighty service providers haven't updated that list of things that "don't" use BIS.
By all means (and I recommend it), monitor your data and costs and (if you're on contract) activate a "data inhibitor", but so far I have not incurred additional data charges on Opera Mini 6.5.