To cite UScensus2000 in publications use:

Zack W. Almquist (2010). US Census Spatial and Demographic Data in R: The UScensus2000 Suite of Packages. Journal of Statistical Software, 37(6), 1-31. URL http://www.jstatsoft.org/v37/i06/.

Corresponding BibTeX entry:

  @Article{,
    title = {US Census Spatial and Demographic Data in {R}: The
      {UScensus2000} Suite of Packages},
    author = {Zack W. Almquist},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {37},
    number = {6},
    pages = {1--31},
    url = {http://www.jstatsoft.org/v37/i06/},
  }