Reply All on the BlackBerry does work like it does in Outlook. If the address you are sending from is different from the address you received the email on, you will be included as a recipient.
For example, all email accounts in my company are our initials (e.g.,
). Our first names are set up as aliases for our accounts (e.g.,
). If someone sends me an email to the latter it appears as a recipient in a Reply All because my account is set up as the former.