people were saying exactly the same when the 3gs was released!
Apple know what they are doing, this will sell. they must know by itunes sync etc how many people still have a 3g/3gs (like me and my partner) and wanting to upgrade!
i had got my 3gs, then when the iphone 4 came out i wanted it but wasnt willing to pay my contract up.
now the time is right for me to upgrade, ive looked at the specs and compared to the 3gs they blow it out the water
i never for one minute expected a change in design for this gen, apple had spent so much time and money on getting the iphone 4 right with the antenna issues, the time spent on trying to get the glass white without light leaking through etc
so taking that into consideration it made sense they stuck with that, it then gives them longer to 100% get the next redesign bang on the money without running into the any wide spread issues as the iphone 4 had