We won't know for sure obviously until somebody seriously benchmarks it, but if Apple's benchmarks are accurate and Barefeats is correct about the 9600M being 43% better than the 8600M, the new integrated 9400M is probably comparable to the ATI X1600 found in the Macbook Pros of old. I've been looking at various Mac-related benchmarks to be sure. There are probably some things that the X1600 is better at and other things the 9400M is better at. I've changed my stance on it, and it will probably be a decent moderate gaming solution.
Anything is better than the awful X3100. Even the old GMA 950 runs the Leopard GUI and older games like Quake 3 better than the X3100!! So the X3100 gets 13 FPS in Quake 4 instead of 8. Big unplayable deal. The GMA 950 gets better FPS in playable older titles and also in things like the spinning wheel, expose, genie effect, dashboard, etc. I feel bad for all those with current Macbook Airs and plastic Macbooks stuck with the X3100. I'd rather have the Rage 3D Pro from my old clamshell! (slight sarcasm)