Having used SnapperMail for about a year, I concur that it is an excellent app. However, like you, I have hands-on familiarity with (and addiction to) Blackberry push email. Indeed, I purchased the 7100t, but took it back- not ready for prime time; then grabbed the Treo 600.
Once I got the Treo, research led me to the "Chatter" app. Bottom-line: a one-time $25 purchase price yields that Blackberry-style experience. As soon as the mail hits your server, it's pushed to the Treo. And, as you delete from the Treo, that too is mirrored at the server, preventing the duplicate review syndrome.
This is one instance where it pays to read the highly helpful manual (pdf). Home page =
http://imchatter.com . I forget how many days you get for the trial. It was so fantastic, I coughed up the dough within 48 hours. Developer is tops, and highly responsive to customers.
I adore it! Two things though: your mail server must be IMAP, and it must relate to the IDLE command. See the forum for Marc/Chatter at mytreo.net >> forums >> Marc/Chatter, which has a list of known OK servers. So far as I can tell, if the server is of the IMAP flavor, it probably does handle the IDLE doohickey.
Happy holidays!
PS. Almost forgot: there is a way that Chatter can also double as a universal instant messenger client. I've not yet investigated this personally, so I'll leave it at just alerting you to the feature's existence.