installing the latest R version (2.13.0) on Ubuntu 11.04
I suspect the CRAN mirrors do not yet have a line for 'natty'. Just make it
deb http://cran.xl-mirror.nl/bin/linux/ubuntu maverick/
for now and you should be fine. This would have been a good question for the r-sig-debian list which is for questions regarding R on Debian/Ubuntu.