Corpora have had tremendous influence on the compilation of dictionaries. It is now no longer purely up to lexicographers to decide what words should (and shouldn't) be included in a dictionary. The process of dictionary compilation has become more scientific and objective now that we are able to assemble and analyze large quantities of naturally occurring data. Not only do corpora give us information about how frequently a word or phrase occurs, they also give us insight into patterns of usage.
Publishing houses now have access to commercially available corpora such as the British National Corpus (BNC), as well as their own in-house corpora. For example, dictionaries compiled by Cambridge University Press are informed by the Cambridge English Corpus, which currently totals 700 million words and is constantly expanded.
There is a useful, non-specialist overview of dictionary developments in the Bangkok Post article (2002) The Dictionary Revolution.