The folks at GRAIN (Genetic Resources Action International) have released an eloquent communique debunking the biotech industry's affirmation that GM (genetically modified) and non-GM agriculture can co-exist: Confronting Contamination: Five reasons to reject GM co-existence

Apart from possible health and environmental hazards, GRAIN warns that GM contamination will further corporate control over agriculture:

Alliances and mergers between the biotech seed industry and agribusiness will take off, giving rise to tightly controlled “identity preservation” systems, where farmers grow particular varieties under contract to corporations dictating what inputs they must use. These identity preservation systems, whether for non-GM or “value-added” GM crops, will be based on certified seed. Meaning, in order to “guarantee” the identity of their crops, farmers will have to grow their crops from seeds purchased from the company, leaving no room for seed saving or exchange. Farmers growing farm-saved seed will have to sell their crops outside of the non-GM stream, unless they can find informal local markets.

Want more info? There's a long version.