Having been recently studying a bit about the language acquisition device, I’d jump at the chance for Spanish (or German, Chinese, Hindi, whatever) immersion preschool if I had a kid.
Warning: above link goes to theory by Noam Chomsky. Do not operate heavy machinery or a right-wing hegemony while reading the above article. 
Hiya.
I was sent to french immersion school from kindergarten until I graduated. (my mom is quebecoise). I have never once regretted it and in fact consider myself extremely lucky for the opportunity. I was even able to do an exchange program in high school and learned more french in 6 months than the entire 12 years previous. To this day when I speak french people assume I am from Montreal and I graduated over 10 years ago.
That being said, not all kids can hack it. We started in kindergarten with 2 classes of 40 kids and only 12 or so graduated with a bilingual diploma. Some kids just aren’t good academically and doing another language on top of regular school is sometimes too much.
On the other hand, I spoke close to no french when I started and never remember feeling like I didn’t know what was going on. The teachers at school spoke french, it was a normal thing. I never personally felt like it was too much or that I was missing out on anything.
If I ever accidentally had kids, they would go to immersion school and have to take another language on top of that.
And if that’s not possible, a nanny who speaks the target L2 can probably be just as good, and maybe even better for affective reasons (no embarrassment or peer-pressure,etc.). Many kids in the richer neighborhoods* are learning Spanish (or Tagalog, etc.) even without the parents’ knowledge, because they’re both working and don’t realize how critical the bonding is that goes on while they’re out.
The girl “Genie,” mentioned in the Wiki article above about the language organ, was not only without exposure to language until she was 13. She was without any real human bonds (maternal, paternal, sibling, or otherwise). As a result, she couldn’t really learn ANY language.
*My financee was a nanny for such a family, and as a result, their children are now pretty fluent in Spanish. In turn, I got pretty fluent in “Barney” and “Teletubies,” as a result of the DVDs she always brought home.
[Disclaimer: I only skimmed the first 20 or so posts before responding.]
I would say go for it. I was in a similar program when I was younger. We started French immersion in Grade 1, and then English was gradually worked in (1 class a day in Grade 3, to half the day in English by Grade 5). This was in Massachusetts in 1986, for the record.
There were a lot of concerns from parents about when their kids would learn to read in English (not mine, since I was reading by age 3), but all of the kids were reading English at grade level by the end of Grade 3, as I recall. There were concerns about helping with homework, too – but we (the kids) always understood the directions before we left for the day, and I believe there were quite a few communications home to the parents, too.
Without hesitation (and I say this as the parent of a 2 year old), I’d encourage you to do it. I’m still fluent in French, although I did bolster my Grade 1-6 French education with 2 years in high school and quite a bit more in university. (It’s relevant that I still remembered enough French to pickup in Grade 11 where I left off (in a 300-level course at Boston University, but that’s a different story)!)
As others have posted above, knowing a second language opens doors. Chances are excellent that your child will be exposed to more Spanish, at some point in their life, even if it’s only during Spring Break in Cancun. 
Thanks to everyone who has responded, I especially like hearing from people who have done it themselves!