I installed GWT Designer 1.7.2 on OS X under Eclipse 3.2.2 and 3.3RC4, I also installed the latest WebKit and configured it according to your instructions.
In general, it's working very well. Nice stuff!
I do see one annoying little bug though. When I activate the "Test/Preview" feature, it's window is rendering mostly offscreen to the top left. About 60% of the window is not visible. There is no way to drag the window down to make it visible. And if I click around on that window, Eclipse crashes hard.
I tried this on two different Macintoshes and with clean installs of Eclipse. It does the same thing in all cases.
Ciao,
Rick