HippieLawChick: While BigLaw might help build the skills needed for a successful Public Interest career, it might come at the expense of opportunities to build useful connections for those jobs. At an admitted students' day I attended, I sat at lunch with the school's director of public interest law. Some students asked about doing biglaw for a few years, and she said that it was very possible but harder to do, since a lot of public interest jobs are obtained by connections (not necessarily a problem). Also, a lot of public interest employers want to be sure that you are really committed, and not just burnt out from the longer hours. I would say that you should build a relationship with some of your future potential public interest employers, and make sure you have a good exit strategy before you go in to a big firm.
Also, while my experience in the matter is limited, the first year associates I talked to at two of the big NYC firms told me that they are expected to use all of their vacations. These aren't particularly lifestyle firms either.