Well, it can't hurt anything, right? But like Diane B. said, there are so many other things you can do to help stimulate your child's intelligence. To be honest, I did not play much classical music for my daughter as a baby - I certainly did not put headphones on my tummy when I was pregnant in an effort to pipe it into in utero! I did have a number of baby lullaby CDs however (including one with classical music) that we got in the habit of putting on to help her sleep and she still likes them on at 3 years old now. But we are also actively engaged with her - we played with her on the floor, we took her for walks, we went places and talked to her, we read her bedtime stories (and of course still do!). I ate as healthy as possible while pregnant, took an omega 3 fatty acid supplement along with my prenatal vitamin, and we do the same for her now. She loves playing with puzzles, building with blocks, and often just trying to construct her own contraptions from whatever is laying around.
We have her in a preschool class this year and just had our first parent-teacher conference last week. The teacher told us how amazed she is at both her intelligence as well as her social skills, plus her enthusiasm for all things school. According to her, she is showing strong language, pre-reading and math skills - she has an impressive vocabulary for a 3-year-old, as well as a sense of humor, and is an active confident participant in group activities (I apologize if I seem to be boasting, I'm just reiterating the teacher's report). She also gets along beautifully with the other children and takes pride in being helpful.
So I don't know if it's the music, or just the combination of everything. It does make sense when you think about it, and studies have shown that children that receive instruction in music (piano lessons, etc.) do better in math and other areas academically. But I also think it is important to have a well-rounded child, one who has empathy, kindness and respect for others, and is not just smart. I've been told by some people that my daughter might be gifted, but I can't be sure. It's hard to test as this age. But as her teacher told me when I asked her about it, she said even gifted kids at this age need to play, and playing is often the best way for them to learn anything.
p.s. DD's favorites right now seem to be Dave Matthews Band and Jack Johnson. :)