My guess is that we WON'T see the kind of open playtest with free downloads that Pathfinder had. My instinct is that to participate in the 5e playtest, you'll need a DDI subscription and you'll need to sign an NDA. But we'll see!
I doubt it will be exactly the same as the Pathfinder Open Playtest (but that playtest had the OGL, so the rules were already "open" anyway).
We don't really know if the open playtest is going to be public docs or hidden behind and NDA (they just haven't told us which way it will be yet).
The Friends and Family (and the D&D XP) playtest is definitely under an NDA.
As for needing DDI to playtest, um no. I am currently a 4e playtester for WotC and half my group has DDI and the other half does not. In fact one of my guys uses Windows Note Pad as his character sheet.
So, unless there is a radical change in policy, no you will not need a DDI subscription to playtest. You will need a wizards.com login (they are free) to post your playtest results.
As a side note, can we all just stop borrowing trouble we don't own? The assumptions on this board and in this thread are painful to read. I know there is low information right now, but good Greyhawk gods people its a little early to say its the Spellplague/Twin Cataclysms/Armageddon right now.
The Fear, Uncertainty, and Doubt (FUD) is thick and people are running around in circles and saying the "Crystal Spheres are falling, the Crystal Spheres are falling."
Everyone take a deep breath and in a month or two people will get their first look at what is really going on mechanically and then you can run around in circles dodging (only if you have the feat) the shards of your world shattering. Or, you can see this as the first baby steps towards a potentially better future of gaming.
This edition has really become a Rorschach test for whether you are glass half empty or half full kinda of person (and there are a few that see it just shattered on the ground with bloody footprints from where they just walked over it without shoes).