That's weird. I checked the repository at /Externals/SDL/ and the 32bit folder has SDL 1.3 included while the x64 folder doesn't. The reason for 1.3 not being included for x64 may be compatibility, since 1.3 is not finished yet. You could try compiling SDL yourself in x64 and see what happens.
Windows 7 RC Build 7100 x64 || Windows Vista SP2 x64
Intel Core 2 Duo E8500 (3.16Ghz)
ATI Radeon HD 4870 512MB (750Mhz/900Mhz)
OCZ Vista Gold 4GB RAM (2x2GB) DDR2-800
Asus P5QL-E
OCZ GameXStream 600W
Intel Core 2 Duo E8500 (3.16Ghz)
ATI Radeon HD 4870 512MB (750Mhz/900Mhz)
OCZ Vista Gold 4GB RAM (2x2GB) DDR2-800
Asus P5QL-E
OCZ GameXStream 600W