IMAP IDLE will be unreliable on almost any cellphone if you try to write your own client:
1. IP address can change in celltower changes.
2. Going in and out of coverage can cause re-obtaining of new IP address.
3. Most cell carriers have a connection timeout, so your IMAP connection can be closed after 120 seconds.
That's why BlackBerry uses their system; it works around these kinds of things. One possibility is Nextel, which allows static IP addresses on their phones.
If you want reliable wireless email that doesn't hang, doesn't get stuck, keeps successfully retrying, doesn't timeout, doesn't display error messages, just works when you want it to, guaranteed reliability, long battery life without the overhead of re-handshaking on every celltower change, IP address change, and compensates for fluctuating IP addresses, you need a push mail server such as BlackBerry or Goodlink or another mobile-friendly system. That costs extra money, and that is why BlackBerry plans are more expensive than other phones, due to the additional server infrastructure.
You can manually check email reliably on a Treo, but you cannot have reliable push email without a BlackBerry plan or a Goodlink plan or another dedicated pushmail system. Plain IMAP doesn't cut it, unfortunately, in a mobile environment with frequent momentary disconnects and IP address changes...
Industry standard is not necessarily best on a wireless device... That's why IMAP is moved to the BWC server, not IMAP directly on the device.
A technically minded person can write your own Linux or Windows Server software that simulates IMAP IDLE support and forwards new messages to the direct push-email address.