He has a flow meter that takes care of point 3, and indirectly point 4
Yes, I missed that -- makes that part easier!
In addition though, it would be nice if there was a way to interface the netbook to the POS system
Yup! Although most cash registers really don't know how to send a message to "please update the database to add X credits for card Y" so in reality, I think most users of such a system wouldn't benefit.
In fact, you'd probably be better off making the netbook *be* the POS system ;-)
Regarding backups, if the cards store value across nights, I would agree. If the system is available only for a night (say, it's a rental or similar), and goes down, unless you keep a second netbook on hand with ability to restore from (near-real-time) backups, the backups of the cards won't help much. If the idea is that this is for a party rental store or whatever, then I'd probably want to ruggedize everything and have a "hot backup" just in case. Cost becomes a factor, too.
Man, this sounds like fun. I wish I was the one building this system :-) :-)