After successful installation of rgdal on a Mac, trying again on an older MacBook Pro running OS X 10.5 (Leopard). Upgraded to the latest R (2.13.1), and followed all of my previous steps. But on “R CMD INSTALL…” got an error telling me that the compiler couldn’t build executables. So assumed that I needed a newer compiler and tried to find the last version of XCode to run on Leopard. No luck. Gave-in and took the time to upgrade to Snow Leopard (10.6) and accompanying XCode. Then tried the rgdal install again. Success! But wait: tried require(rgdal) in R64.app, as before, but getting errors about rgdal not being for x86_64, and rgdal is not available, etc. So tried R.app (the i386 app) — and it worked.
Assume the difference is between R 2.11 and 2.13, or that initial R install was on OS X 10.5. Regardless, it seems that the most important thing is to get rgdal to cleanly build from source. Then just use the R app that works.