After reloading my contacts.cdb and Agenda files from the '800 onto the '900, I set off for a jaunt in FSR, which involves turing off the phone for the flight. On restarting it, the contacts were (thankfully) intact, but my diary and todo list had vanished!
Bummer, especially since I had spent about an hour re-assigning categories and colours in the diary.
Turns out that there are two copies of the Agenda file; one in system\Documents and the other in Startup. Both copies are identical!
By copying my old file to both places, the diary and todo are up and running fine.
Anyone like to suggest as to why there should be two identical copies?
Sounds a bit like the old "2nd FAT" idea from MS-DOS, in that there was a second copy as a backup, except because the transaction write was so closely coupled, generally you could rely on both copies being corrupt before the system noticed and shut down :(