Woo Hoo, been using nokia mail for Exchange for around a month now, it's available for most E and N series devices.
Only needs exchange server with webmail access.
It has a always online mode which doesn't poll it just stays connected to the exchange server during selected peak hours.
the only traffic above the headers being sent, it the keepalive every 20 minutes and if you actually choose to read a mail. so 500kb a day in traffic is what I see on my account.
one little glitch is that you need to provision the VPN solution and drop the cache servers otherwise the keep-alive doesn't work properly and drains the battery.
Over and above this, it'll sync contacts, tasks and calendar... pretty much same as blackberry.
Hope this helps someone.
D