Brighthand takes a look at why some PDAs will be upgradeable to Windows Mobile 5 and why others will not. Much of the explanation has to do with persistent memory. Most Pocket PC devices today will lose all their data if the batteries run dry (because RAM which needs to be powered at all times is used to store applications and data). To address this issue, Microsoft has reworked how memory will be used in Windows Mobile 5.
In the new OS, RAM will only be used for running applications. ROM will be used for storage. Because of this change, a lot of devices today will not have the necessary ROM to support Windows Mobile 5. As a result, these devices will not be upgradeable to Windows Mobile 5.