I got a sudden hard Reset on my WM5 upgraded X50v. My PIM (calendar, contacts) information are still there, but all of my add-on software loose their settings and entries in Start Program Menus, samething for Today-Plug-ins. They become unusable.
Please have a backup handy (as before!) on your storage card.
WM5/PS - Persistent Storage only helps in case, you drain your power completely. When you recharge your unit, everything seems to go back to normal unstead of a full-hard reset condition. (My speculation, can somebody validate?)
If you plan to sell your unit and try to clear your PIM data. Use a product likes PocketInformant, which has an option to do that.