IS there ant unique id exist for contact/task/calendar items

I used combinely the entryId and storeID to create unique id .

When Iused function GetFolderFromID(storeid,entrid) to get the item, it
return the error message that item is not ready.

Does any body why this error message is being generated or what is the
unique id of outlook2003 items.

Is EntryId is enough for uniqueID??

Thanks i nadvance