I've got a small problem rendering cloudy particles with fluids. The particles opacity is driven by an expression, particleCloud as shader connected to particleSamplerInfo for transparency/life transparency. it works fine if I render my particles alone, but if I render those on top of a fluid I see round shapes of the particles even if the opacityPP is close to 0 (<0.001) Maya2008x64 extension2 on XPx64 SP2
On the fluid node try enabling volumeSamplesOverride and increasing the volumeSamples. This improves the integration between fluid and particle densities.