Second, I didn't imply that prior versions Mac OS X didn't have kernel protection, I implied that prior versions of Mac OS didn't have kernel protection. This is indisputably true (see: Mac OS 9). Personally, I find Windows / Mac OS parallel surprisingly close here: Windows ME is to Windows XP as Mac OS 9 is to Mac OS X Cheetah.
Third, UAC (User Account Control), the access control introduced with Windows Vista, is almost entirely unrelated to kernel protection (except that UAC would probably be pointless without it). The problem UAC tries to solve is "users running as an administrator too often", not "the kernel isn't protected from user programs". In other words, it is Windows' answer to sudo, not a fundamental change to the Windows kernel.