My wild guess here is that AMD knew there was fair, but not an overwhelming amount of improvements capable out of the RV770 core. So they poured the majority of engineering resources into RV870, which there are now rumours of an earlier than expected release. Still though, I bet the HD4890 will be a worthy successor to the HD4870 in a couple ways. However, there is a good chance that i'm lost out in left field here too.