For some reason R is not happy with its 64-bit cousin when installing source packages:

* installing *source* package ‘XMLSchema’ ...
** R
** inst
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** installing vignettes
** testing if installed package can be loaded
*** arch - i386
*** arch - x86_64
/Library/Frameworks/R.framework/Resources/bin/R: line 259: /Library/Frameworks/R.framework/Resources/bin/exec/x86_64/R: Bad CPU type in executable
/Library/Frameworks/R.framework/Resources/bin/R: line 259: /Library/Frameworks/R.framework/Resources/bin/exec/x86_64/R: Unknown error: 0
ERROR: loading failed for ‘x86_64’
* removing ‘/Library/Frameworks/R.framework/Versions/2.15/Resources/library/XMLSchema’

Quick hack to get things moving along: throw out the 64-bit binary, located at:

rm -rf /Library/Frameworks/R.framework/Resources/bin/exec/x86_64/