There is a KB that covers an HTTP message delivery problem on 64-bit Vista.
936497 BUG: Error message when you try to send messages to a Message Queuing queue on a computer that is running a 64-bit version of Windows Vista: "404 not found"
The problem occurs when Internet Information Services (IIS) is configured to run in 32-bit mode on a 64-bit version of Windows Vista. 32-bit IIS is expecting to interact with 32-bit MQISE.DLL but that file does not exist in the %WINDIR%\SysWOW64 directory - there is only a 64-bit version in the System32 directory.
The workaround is to find a copy of MQISE.DLL for 32-bit that matches the version of the 64-bit MQISE.DLL and copy it to the SysWOW64 directory. Note that this means you will need to personally maintain the DLL as there is no guarantee that Microsoft's future hotfixes and service packs will look after the 32-bit DLL for you. Just make sure when you order Vista hotfixes for MSMQ that you request both 32- and 64-bit versions. Currently there are no Vista hotfixes for MSMQ so no worries just yet.