The iPhone 3G S went on sale last Friday around the world. Fortune's Apple 2.0 says that Piper Jaffray analyst Gene Munster has issued a report today estimating Apple having sold 750,000 iPhones over the weekend.
Update: Apple just announced that they sold over 1 million iPhone 3G S over the weekend.

This is quite a bit more than the originally predicted 500,000 iPhones.

Apple has sold within the first 3 days 1 million iPhone 3G last year at its launch. Still, if the the 750,000 are true that would be huge as it mostly would mean new iPhone customers. Most current iPhone owners are stuck in their current contract runtime and would have to pay premium for the new iPhone 3G S.
I have talked to several iPhone 3G owners and they all stay away from the iPhone 3G S because of the cost and that the couple of new features/differences are actually not tempting enough.
More iPhone 3G S News.