package installation fails only for "sp"

I have escaped Splus for Windows (mostly) and have started using
R (v 2.3.1 on i686 redhat).  Installing packages has been routine except
for "sp" (classes and methods for spatial data).  I get the following error
message

 > install.packages("sp")

Warning in download.packages(unique(pkgs),destdir=tmpd,available=available,:
            no package 'sp'  at the repositories
 
Clearly I am missing something (probably obvious), but can someone 
enlighten me
as to why the behavior of this package installation is different from 
others?  sp is
listed under "Contributed packages" at the R-project URL.  Thank you
for your time.