We know that the demand for the iPhone 3G is huge. TechCrunch reports that their sources tell them that Foxconn is now producing 800,000 iPhone 3G units per week for Apple.

Foxconn's facilities would be running above capacity to achieve that output. Over the course of 12 months Apple would be producing 40 million iPhone 3G devices at that pace.
Since several markets are sold out in Europe and Apple is adding more markets that number of iPhone 3G could actually be necessary. Although earlier estimates by analysts saw the yearly production at around 25m.
More details on TechCrunch.