When you send your mail to your mail server (ie: yahoo.com, gmail.com, hotmail.com, etc.), you send out the following as ordered below:
1) Your e-mail address
This lets the server know who you are sending it from.
2) Addresses to send the message to
These e-mail addresses include all To, CC, and BCC recipients. There is no disctiction between which addresses are To, CC, or BCC. The mail server uses this information in order to forward your message to your specified recipients
3) Your message. This is what is passed on. E-Mail addresses that the message are sent to usually appear in what is called the "header" of the message. This "header" is generated by your e-mail client (Outlook, Outlook Express, Webmail, etc). Since mail servers generally do not use these header information to deliver mail (they do use it to filter spam), not all the recipients e-mail's need to be listed. By not listing certain e-mail's in this header, you may send your message to a variety of people, that other recipients do not know about.
Okay, now to answer your question.
It is not easy to track down the BCC recipients of an e-mail message you received. In order to do so, you would need to contact the administrator for the sender's domain.
That is, if the mail is sent from amadeupaddress@hotmail.com, then one would have to contact the mail administrator for hotmail.com. Abuse@WhereverItWasSentFrom is usually the best guess on where to obtain information on the BCC recipients.
Now, even if the administrators of that domain keep such detailed logs (which most don't), it would be almost impossible for you to get the information out of them. It usually takes something like.. um.. lets say.. oh.. A COURT ORDER!!!!
So, in not so many words. Yes, under extremely unlikely circumstances, it is theoretically possible to track down the recipients. For the most part, however, most people either just live not knowing who else got the same message, or simply call and ask the person who sent it to them.