Today we have released Service Pack 2 for Exchange Server 2003 to the web, including the new Direct Push Technology. To get more information and download the service pack check out this link: http://www.microsoft.com/exchange/downloads/2003/sp2/download.mspx
To find out more information about device compatibility with Exchange Server 2003 have a look at this link: http://www.microsoft.com/exchange/evaluation/features/owa_mobile.mspx
Please note you will need the Messaging & Security Feature Pack for Windows Mobile 5 devices to enable this functionality on the device side. Check with your device OEM or mobile operator for more information about when they will have this tested and available.