Some of you might have already figured it out, or read about it in the change log I posted about the Upgrade thread, but if you haven't I thought this was important enough to discuss.

PM's now work more like email. When you send a PM to multiple people (using the To: field) everyone will know who got the message. If you use the BCC field, those recipients will remain private.

From the change log:
Full BCC Support in Private Messages
A number of you have discovered this by now. Like in an email, BCC (blind carbon-copy) allows you to hide the full list of recipients from each recipient. In previous versions of vBulletin, the “to” field acted like a BCC field. Now, the “to” field acts like the field in an email; each recipient will see all the names in the “to” field.