From my read of things, there's two different time windows here:
90 days -- They wont have a payment mechanism at first, so only completely free apps will probably be on the store for the first 90 days.
30 days -- The store will have a 30 day "try before you buy" option. I assume that means that once the first 90 days have elapsed, and you can start to _buy_ non-free apps, you'll first be able to try them risk free for 30 days (like an initial demo mode or something).
That doesn't mean "for X days you can download EVERY APP and keep it for forever". It's just two different time windows.