Sys undefined?  Then you've come to the right place.  After some considerable sleuthing I happened upon this thread, in which someone mentioned compression.  That, coupled with viewing source and loading the scriptresource.axd entries directly in the browser to which I got some nice "padding is invalid" and "index out of bounds of the array" errors, led me to an entry in web.config that was in the sample config that comes with the Ajax Futures installation, but was not in my web.config:  Check for a section called:

<system.web.extensions>
<scripting>

and add a

<scriptResourceHandler enableCompression="false" enableCaching="true" />

That fixed it for me.  I'm not sure why compression wasn't a problem in the older CTP's, but I just LOVE that later versions of stuff breaks.  I know I know.  It's only a CTP, but still...:)