Apple has discontinued support for Windows Vista and Windows XP for Bootcamp on the new MacBook Pros that began shipping last Thursday.
«MacBook Air (Late 2010) and MacBook Pro (Early 2011) computers support 32 and 64 bit versions of Windows 7 Home Premium, Windows 7 Professional, or Windows 7 Ultimate.
Note: Windows XP and Vista drivers are not supplied for these computers and are not supported. »
I don't quite understand why Windows XP is no longer supported with how well its SP3 version works, and there are still many people reluctant to switch to Windows 7.
Source: iClarified