Currently, there isn't an option for 64 bit ArcGIS Desktop. -- many of ESRI's competitors do offer this. In face Autocad Map has had a 64 bit version for a few years now. The datasets we use in GIS are becoming larger and larger (such as point cloud data) and our little-bitty 32 biut clients choke & crash all the time.
This idea has been merged with 64 bit ArcGIS Desktop.