Yes. It is not particularly hard. It can be a bit inconvenient. You
will need to run fluid from the terminal with DYLD_FRAMEWORK_PATH set
to the build of WebKit that you want to use. For example, you have a
SSB called TestWebKit then depending on your choice of shell,
something like this will run it with a WebKit nightly.
DYLD_FRAMEWORK_PATH='/Applications/WebKit.app/Contents/Frameworks/10.5/
WebKit.framework/Versions/Versions/A/WebKit' /Applications/
TestWebKit.app/Contents/MacOS/FluidInstance
That being said, you could just install the Safari4 preview if you
want most of the nightly's new features without being quite so
bleeding edge.
Rob.