If you can use IMAP IDLE then you have push email in practice, as long as you're not after a push-system with notification through a separate channel.
With IDLE there's very little traffic. It's ideal when you receive mail only now and then. Keeping a 3G connection up all the time won't kill your data budget. However, if you get mail all the time it's understandable that some will then prefer to get them in chunks, i.e. going for poll instead.

Point is, nothing fancy is needed.. use an IMAP server with IDLE support (e.q. courier), get a client with IDLE support (e.g. Thunderbird .. oops.. we don't seem to have any Maemo mail clients with IDLE support yet. )