Have a look at # 5 of the Apple > About the SD and SDXC card slot support article.
Does the SD slot work with cards that exceed 32 GB?
Yes. However, most media manufactures preformat the media using common block-and-cluster sizes that do not
approach the theoretical limits of a given file system. Most SD cards use the FAT32 file format. Preformatted
FAT32 SD media is commonly available up to a capacity of 32 GB. Media that exceeds 32GB usually uses the
exFAT file system. Some smaller capacity cards use the FAT16 file format. Preformatted FAT16 media is generally
available up to a capacity of 2 GB. To determine which file system you are using with Mac OS X v10.6.5 or later,
insert the media into the SD card slot, go to the Apple menu, select About this Mac..., click on More info..., click
on Card Reader in the hardware section, and locate the File System field.