I am a bilingual reading specialist in a predominantly Hispanic school, and I teach in a dual language classroom. My son (who is as white as they come!) understood some Spanish and spoke a little since I exposed him to it since birth, however, since he was old enough to begin a dual language preschool program he has learned so much more. His understanding of the alphabet (in both languages) and pre-reading skills have just exploded as well. Yes, they are exposed to Spanish on tv and from many video games and toys, but they pick up so much more when immersed in a learning environment with the opportunity to dialogue with real people.
The opportunity to learn an additional language (making your child so much more marketable in the future) aside, a dual-language environment provides a boost in learning experiences in the native language as well. Research proves that children who know multiple languages understand their own language better, reading and writing from an earlier age. Also, the connections that they make between languages helps expand their vocabulary and make brain connections that instruction in one language could not duplicate (think about being taught Greek or Latin to learn root words).
From my experience as a native English speaker learning Spanish as a second language, then teaching Spanish, as well as the experience of my son, I would say without a doubt-Go for it! Besides, if it doesn't meet your child's needs, you could always switch later. It could set the child back a bit to switch, but by 4th grade, should be well on track. Hope this helps!