i dont know about RC1 since i havnt used it, but the one included with bootcamp works fine on the second beta release. you just have to manually install it. there is a way to unpack the cabs or whatever in the driver installs that are included on the bootcamp disk. then you can install the driver manually.
these are the instructions for the mini, but they are probably the same for the iMac...or at least give you a push in the right direction.
(1) Boot your Mini into Vista, (2) Insert your BootCamp Driver CD, (3) Close any attempts that Vista attempts to autorun the CD and open Windows Exploer, (4) Browse over to Bootcamp CD and copy the 200MB executable named "Install Macintosh Drivers for Windows XP.exe" to your hard drive, (5) Now run the executable from the hard drive with "/V /a" flags (I recommend doing this from the Run prompt. However you wish to run the executable is up to you but you need to run it with those flags which tell the installer to extract the drivers only. You will need to give it a directory when prompted that you want everything unpacked to.), (6) After everything unpacks you should have a sub-folder called "Macintosh Drivers for Windows XP" somewhere in the directory you gave earlier to unpack everything. Under that folder, look for a folder labeled "sigmatelmm11", (7) Run Setup.exe from that folder and it will install the Bootcamp sound drivers.
they worked in the beta for the macbook pro, but the sound was seriously wonky and required restarts every so often. also the optical out light stayed on all the darn time
