Previously I posted two ways to boot "capable" Macs (other than the Xserve) into 64-bit Snow Leopard: either a) hold down the 6 and 4 keys at boot or, b) edit the Kernel Flag string in the
Apple Core reader Robert Nicholson notes that there's another way to boot into the 64-bit version of Snow Leopard -- by setting the nvram paramters (similar to verbose booting) as noted on My Grotto:
Print nvram settings
# nvram -p
Set boot-args to use 64 bit kernel
# nvram boot-args="arch=x86_64"