To cite the biomaRt package in publications use:

  Mapping identifiers for the integration of genomic datasets with the
  R/Bioconductor package biomaRt. Steffen Durinck, Paul T. Spellman,
  Ewan Birney and Wolfgang Huber, Nature Protocols 4, 1184-1191 (2009).

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Mapping identifiers for the integration of genomic datasets with the R/Bioconductor package biomaRt},
    author = {Steffen Durinck and Paul T. Spellman and Ewan Birney and Wolfgang Huber},
    journal = {Nature Protocols},
    year = {2009},
    volume = {4},
    pages = {1184--1191},
  }

  BioMart and Bioconductor: a powerful link between biological
  databases and microarray data analysis. Steffen Durinck, Yves Moreau,
  Arek Kasprzyk, Sean Davis, Bart De Moor, Alvis Brazma and Wolfgang
  Huber, Bioinformatics 21, 3439-3440 (2005).

A BibTeX entry for LaTeX users is

  @Article{,
    title = {BioMart and Bioconductor: a powerful link between biological databases and microarray data analysis},
    author = {Steffen Durinck and Yves Moreau and Arek Kasprzyk and Sean Davis and Bart {De Moor} and Alvis Brazma and Wolfgang Huber},
    journal = {Bioinformatics},
    year = {2005},
    volume = {21},
    pages = {3439--3440},
  }