I abide by the age-old advice of dressing just a notch more formal than the expected attire of your interviewers, e.g. for Google typical interviewer would be in jeans and t-shirt, so khakis and a polo.
I would add that this will vary by industry and location as well. If you're in the bay area interviewing for a startup, you can get away with a lot more casual dress than if you're trying to get on the IT team for a financial firm in downtown Chicago.
One approach you can use is to ask the HR person over the phone what the culture is like, or to look at their "about us" webpage. These will provide other useful signals as well.