I had the same problem... turns out, Google Gears isn't compatible
with Snow Leopard yet. So, Fluid was loading Gears, which was
failing, but Gmail still thought it was working, so it tries to call
Gears specific Javascript functions, which don't exist.
So, I just went to the Google Gears menu bar settings and set
everything for "Denied".
My GMail fluid app started right up after that.
Marcus