Preen is mostly corn gluten, which is considered organic. There are, however, different types. One is for vegetable gardens and, looking at the MSDS, doesn't contain harmful additives. The other is for "gardens" (I guess this means flower beds?) and is not considered organic.
That said, even the first one has a warning about preventing contamination of ponds, streams, etc. As soon as I see things like that, I tend to shy away. I'm also not sure, based on the label notes (available online) if it kills all weeds, or just the select few listed. I'd check which weeds you're trying to prevent before using it. Further, it works by killing seedlings, so if you're planning on direct sowing, I'd use caution there, too.
The most "organic" ways of preventing weeds are hand-pulling and mulching. And keep weeds from spreading by pulling and/or cutting before they go to seed.