There's a catch.

On the N900 it seems flipclock only syncs the alarms, every second try. I.e. set an alarm, go back to the main screen... If you see the system message "Alarms synced..." it'll sound.

If now you go reset the alarm to some other time, and go back to the main screen, it doesn't sync the alarm, i.e. no info message. Fliplock still shows 8 hours till the alarm, or what have you, but it ain't gonna sound.

If you just go in to set the alarm again, don't change anything and go back to the main screen, it'll show the message "Alarms synced" and it'll sound then at the alotted time.

This is the biggest catch... serious, I also learnt the hard way...