Just to clear up something that wasn't touched on that you mentioned...
When you forward your email through the Gmail setup, it does not come as a forwarded message (ie. the subject does not say "FWD: subject") it more or less "bounces" a copy over to the BlackBerry Internet Service account and therefore delivered to your BlackBerry.
If your new company is Treo favorable, then that would mean that they have POP access set up and therefore the BlackBerry Internet Service can be set up to poll it and deliver new email to the BlackBerry. The downside of this is that there is a 15 minute delay between pollings. Some people have set up a forwarding rule in Outlook to get around this and it works just like the Gmail forwarding.
Your other option for the work account may be setting up the mail connector method on your work computer. This would make it real time as well. It requires either a Domino server or an Exchange server, and that you keep your work computer on at all times. Some people like this idea, some people hate it. I personally never turn my computer off at work so it wouldn't be that bad for me to set up if my company didn't have a BlackBerry Enterprise Server.
Hope this helps add a bit more info to the pot for you