I think the readGDAL() function from the rgdal package (http://cran.r-project.org/web/packages/rgdal/rgdal.pdf) can do the trick, since grib2 is a GDAL-supported format.
If not you can also use a dedicated free software to open data before importing it to R: http://sourceforge.net/projects/gribcs/.
Convert grib to netcdf using the command line function ncl_convert2nc from ncar command language use package ncdf in R to import the netcdf data.
or,
rgdal also can do it, but is less flexible and requires more care and detail than ncdf or RNetCDF - and depends of your GDAL/rgdal built including the GRIB driver.