In the end that is the answer. If I want the Message Icon to hold only my BIS email (my "name"@blackberry.att.net) not my Yahoo mail as well I can't. I have no problem with the Messages inbox having my SMS's as well as my email... that doesn't bother me (because the envelopes are different shapes helping to differentiate between email and SMS's.) It's the double email accounts in there that can get confusing.

It also looks like Yahoo mails IMAP'ing through BIS isn't syncing properly either - since what is on my Blackberry does not match what is on Yahoo Mail on the web. I can't fix that either, that's Yahoo's "issue." I did contact them but they were no help whatsoever.