Did you just purchase Prowl only to find out that it does not work on jailbroken iPhones? The creator of Prowl is aware that jailbroken iPhone will not properly register and thus push notifications will not work on most some jailbroken iPhones.
Prowl is an application that lets you push pretty much anything to your iPhone. As long as you are using Growl on your mac or windows(in beta) Prowl will push all of your Growl notifications to your iPhone. The possiblities are endless!
Most people who have jailbroken their iPhones have mobilesubstrate installed and probably SBSettings. In order to get Prowl push notifications working on your jailbroken iPhone you need to reboot your iPhone into safe mode and then launch Prowl and let it register your iPhone.
You can do this by opening SBsettings. Hit Power. Then press Safe Mode. Once you are in safe mode just open up the Prowl app and it will register your iPhone. You can now reboot into regular mode and growl push notifications will be working.