I think that by middle of 2005, there will be a much better POP client for BlackBerry, for those users who want to integrate the BlackBerry with push email.
The problem is nobody bothered to develop one, because BlackBerry email was great enough (Especially if you are a BES4 user using an Exchange Inbox -- wireless Outlook PIM/email synchronization is becoming a major selling point of BlackBerry)
However, BlackBerry is indeed a much more open platform than SideKick/Hiptop. It is powerful enough to allow you to write a POP client without needing a signature key from RIM. You can develop a POP client for BlackBerry without even telling RIM about it. (Unless, of course, you want to 'hook' into protected API's the vibrating/alerting system) If you are a developer, please visit the BlackBerry Development Forum
There is Reqwireless EmailViewer from http://www.ReqWireless.com
which is the currently best POP/IMAP client for BlackBerry. Any new programs would have to have at least a feature or two that EmailViewer does not have... such as maybe full integration with the RIM Messages list, and full background alerting.
There was also Sparrow email, which was a great little email program for RIM950, but was not ported to the newer BlackBerries.
The good things is that BlackBerry development is exploding now. At the start of 2004, there were only 2 chat programs. Now, at end of 2004, there were over 15 chat programs.