If you launch Espresso from the Terminal using this script it will use the WebKit nightly (assuming WebKit is installed in your root Applications folder):

 DYLD_FRAMEWORK_PATH="/Applications/WebKit.app/Contents/Frameworks/10.7/" WEBKIT_UNSET_DYLD_FRAMEWORK_PATH="YES" open -a Espresso

Note that this is not a toggle; it only affects Espresso once at a time. An easier way to do this could be via Applescript or something.

Ian Beck, Macrabbit Support