This support page from Apple tells you "which Macs can use the 64-bit kernel in Mac OS X v10.6 and which use it by default." It also tells you how to check your system.
http://support.apple.com/kb/ht3770
According to the page, these Macs use the 64-bit kernel by default in Mac OS X v10.6:
Mac Pro (Mid 2010)
MacBook Pro (Early 2011)
iMac (21.5-inch and 27-inch, Mid 2011)
These Macs support the 64-bit kernel, but do not use it by default:
iMac (Early 2008) and later
MacBook Pro (Early 2008 through Mid 2010)
Mac Pro (Early 2008) and later
Mac mini (Mid 2010)