"Stepping" more or less refers to a revision number of that processor's type. The higher the letter an number, the further that particular processor's archetechture has been refined. So a g0 compared to a b3 is much further along the line in design and testing.
What this usually means with processors is that the processor will run much cooler, have a longer lifespan, process more effeciently, and be far less prone to glitches.
Apparently, the heat difference between b3 and g0 on this particular processor is huge. This is especially useful if you're into overclocking, as there are reports of overclocking the thing from 2.4 GHz to 3.6 GHz with the stock cooler and remaining stable. I'll believe it when I see it, but that sounds amazing. I'd probably think it safer to stop around 3 GHz, and only if there was little or no sign of heating up signifigantly under load.