thats a bummer
heres from the society of barefoot living faq entry about getting things in your foot...
http://www.barefooters.org/faq/12.html
its basically what lady sage said.
I would try to take care of it yourself before going to the hospital for it... see if you can get a friend to dig around in there with some needles, nail clippers and a razor blade. it can be awkward to try to work on your foot, depending on where the sliver is.