i just can't figured out this that you said: "the previous owner of your phone clearly got your ECID on file while 4.0 was still being signed, i.e., before July 15. You're now all set for future versions, and can indeed upgrade to 4.0, 4.0.1, or 4.0.2.", because i bought it on a store, it was sealed, i activate myself the iphone via iTunes, so it wouldn't be loggic that i could only have SHSH for 4.0.2 (the only firmware i think that apple is signing in the moment??)
Your phone must be a refurbished model. I hope you didn't pay full retail for it. But, on the other hand, you got a phone with its 4.0 SHSH on file...so I wouldn't complain.