For anyone that's interested, you can achieve the same 'push' features that Blackberry & Exchange 2003 (SP2) have. I have tested this with the current version of Zimbra and a Treo 650 (also works with a Treo 600) and an email client called ChatterEmail (ChatterEmail is exclusively for the Treo 600/650) from here
www.chatteremail.com - email arrives in the Zimbra and, voila, almost instantly on your phone/PDA.
There may, of course, be other solutions to this but I thought someone might be in the same position as me and needing instant email notification - at a relatively low cost with Zimbra & ChatterEmail.
Regards
Bill