I have a Cingular Blackberry, and use their web based email to catch all of my mail. Basically, I have three email addresses forwarded to my
account. All messages come to my Blackberry. Nextel had something similar when I used them a few months ago.
What doesn't seem possible is to SEND email from your Blackberry and have it send from multiple addresses. For example, when I reply to a message the reply always seems to come from the same email address. ...regardless of which mail account I'm answering.
If you are using BES I think there is a work around. You can set it so that you when you start to reply you can elect to send the reply through either the web client or the desktop client. In that case, the web client can be set to reply with one address, and the desktop (Exchange) client can be set to reply to a different address.
Not the best solution, but I hope it helps.