Your best bet is to stick with the BlackBerry because I honestly do not think Treo can do what you're asking. Also, stay with BlackBerry Enterprise Server, and add the BlackBerry Internet Service (I use this) to have access to your personal email addresses.
With T-Mobile, when you create an email, you can choose what address the reply will be sent to. I have this setup using my AOL account. I can send out an email using my AOL account and no one knows I'm using a BlackBerry LOL. To do this,
1) Go to Message, Compose Email.
2) Highlight "Send Using" at the top of the page. It will say [Default].
3) Click on "Change Option."
4) Scroll to and click on the address you want to use.
5) Compose and send your email.
I think you can set up your corporate address to show that instead of your BlackBerry address when sending emails too.
You WILL need to keep your corporate account to synch your calendar, address book, notes, and tasks, and also for wireless synchronization and filing of your emails using a Microsoft Outlook, Lotus Notes or Novell Groupwise server or Hosted BES. This will allow you to file sent emails in the Sent Folder. Deleted items will show up in the Deleted folder, etc.
To see how to add your personal email, go to http://www.discoverblackberry.com
, click on "Need Help?" in the top right corner of the screen, then choose either the 8700c or r "Getting Started" demo from the left side of the screen and watch the entire video to set up your BIS and BES email on the same device. Be sure T-Mobile allows setting up both options by calling them and asking before you try this.