"Business casual" is one of those phrases that every tech company today says, so it's lost all meaning. (Do they also say "quality is job one" and "we only hire the best"?)
Of the last 3 places I've worked, the dress code has been fairly different, despite always being described as "business casual". So you've really got to ask, or take a tour, to find out what it means to them.
FWIW, the most formal I've seen it mean (in Seattle) is jeans, a t-shirt, and shoes. But I've never worked for a bank, or in NYC, or for Intel Corp, or as management -- any of these things could drastically affect what they mean by it.