contrib.url in non-interactive mode
It is up to you to set a default mirror: we have little idea where you live (and it may not be where your email address suggests). Geolocation of mirrors had been mooted but not implemented (and in a corporate setting is not 100% reliable). The whole point of mirrors is to spread the load: without them there would be no CRAN as the main host would not allow the traffic.
The RStudio CRAN mirror uses Amazon's CDN to achieve a degree of geographic convenience: http://blog.rstudio.org/2013/06/10/rstudio-cran-mirror/
Cloudfront, the service we use, currently has 36 locations around the world: http://aws.amazon.com/cloudfront/details/. Hadley