You can check out the release notes for the assemblies here: http://www.ncbi.nlm.nih.gov/genome/guide/human/release_notes.html
See the notes on Ensembl 59 here: http://uswest.ensembl.org/Homo_sapiens/Info/Index
(Off topic note: is anyone else's favicon for ensembl backwards? Or is it just me? Or is it just the US site and it's some kind of insult ;) ?
for any of you interested in working with Ensembl data mapped to the previous human reference NCBI36/hg18, now that it has been clearly stated here that Ensembl59 uses GRCh37/NCBI37/hg19, the latest version would be Ensembl54. It can be found browsing their archive site, a great resource for people still dealing with old mapped data.