My understanding is that with deflation, people postpone purchases on the expectation that prices will continue to fall, which perpetuates the cycle. Real estate shouldn't be any different...buyers will wait for a better price. I suspect that falling home prices would also leave more people underwater, leading to more bankruptcy, short sales, and tight inventory - not unlike what we're seeing now.
Kevin Yoo - I think you're right that very low interest rates would make for cheaper debt. However, my worry would be that since the PITI is fixed, your cash flow will decrease over time as rents go down. You won't be able to count on raising the rent on a regular basis.
Flipping would become much harder, as you'll have to find a seller who can both find financing and is willing to buy now, rather than waiting for a cheaper price.
Honestly, IMHO, real estate is a tough investment in deflationary times. For a short term fix, I'd try to lock in tenants with longer leases (yeah, I know, tenants have nothing but respect for a lease....) Your income stays the same over the period of the lease, while your expenses go down.