Mapping gene ids is one of the routine jobs for bioinformatics. I was aware of several ways to do it including Biomart.
Update on 10/30/14, a mygene bioconductor package is online http://bioconductor.org/packages/release/bioc/html/mygene.html
Recently I got to know mygene, a python wrapper for the mygene.info services to map gene ids.
I found it very handy to convert gene ids. see a gist below.
To use it, cat input.txt | python geneSymbol2Entrez.py > output.txt
or python geneSymbol2Entrez.py input.txt > output.txt where input.txt contains one gene name in each line. pretty neat!