This is a rare Exchange posting, but with the release of MSFP ROM updates for Windows Mobile 5 Smartphones and Pocket PC Phones, Direct Push is now a reality. However, there are some optimization tweaks that can be done that would extend the battery life of the smart phone devices, plus there are some caveats when you are using firewalls that will terminate the connection too quickly.
See here for the blog entry.
I can vouch that Direct Push works well. I have a Cingular 8125 as my main phone right now. I installed the Imate MSFP ROM (available at XDA-Developers) and started using Direct Push as soon as my hosting provider (LanLogic) upgraded to SP2. Others at my office have the Sprint PPC6700 that has also been updated to the MSFP ROM. In general, we noticed a huge improvement in battery life. My Cingular 8125 now has about an average of 15% battery drain per day with DirectPush enabled and with about a half hour of phone calls. The CDMA-based PPC-6700 loses about 25% a day.