You can't tell anymore by the serial numbers.
When the iPhone is 'refreshed' it is given a new serial number.
Apple does give refurb models, from my source (yes, even though i've only posted 2 times) say's all refurb models start wit ha 5K serial number rather then 8 etc.
you will notice if you have had your phone replaced (Canada) your serial number 99% of the time is now 5kxxxxxxxxx rather than 858 or 8xxxxxxxx etc. this goes for the iPhone 3G, I have not yet asked about the 3GS but I would assume, just like when the 3G came out, apple used NEW product, rather then refurb, as they had none send back to the factory for 'refurbing'.
Apple also notes all REFURB models, get a new exterior casing for the phone, and potential screen replacement's, and also new parts required for actually repairing the device itself.
I still don't get why apple does not get their mac genius team to repair the iphones, in other ways, then just replacing the screen, or iphone itself, the only reason I think they do this, as it is good on time, 15 minute appointment, 5 minutes to do the paper work, 10 minutes to replace and test out the screen.