The initial build for Raring Ringtail did not come with Tcl/Tk being supported. This issue has been addressed and the current release supports Tcl/Tk.
Due to a small configuration error on my server, the amd64 packages for Raring Ringtail did not sync to CRAN. This has been fixed and the 64-bit packages should be available soon on a CRAN mirror near you.
As has been previously discussed, R 3.0.0 requires that all packages be rebuilt against it. Therefore, all the r-cran-* packages found in the Ubuntu universe repository will not work with R 3.0.0. In an effort to rectify this, I have rebuilt a number of those packages against R 3.0.0 and they can be found at the RRutter PPA, but not CRAN. I am still having issues with a couple of packages, but I am working on those.
R 3.0.0 is still available for older Ubuntu releases on my RDev PPA. The plan is to upgrade Lucid, Precise, and Quantal this Friday evening (May 3rd, 2013) as well as many of the r-cran-* packages available in universe. If you do not want to upgrade to R 3.0.0, please remove the appropriate repository from you sources list.