GDAL was having problems with GeoTIFFs in 64bit mode. I tracked it down to an optimization problem, probably bugs in Apple's GCC. The only solution I could find was to enable minimal optimization (-O1 for those that know what that means). Hopefully Apple will address this and other Xcode problems soon. Until then, GDAL may be a bit slower than usual.
Another 64bit problem is with the HDF4 format. Corrupt reading, and segfaults on writing. I was able to get writing to work, but there are still corrupt reads. The HDF4 libraries apparently have some known 64bit issues, which will hopefully be addressed in the next version.