Either way you look at it your average Joe will look at the 6870 and figure it faster than a 5870, crazy naming scheme.
The average "joe" isn't going to have a 5870, people need to stop thinking they will. There a load of people who consider themselves "in to" computers and PC gaming who think the amount of RAM your graphics card has, determines its performance as well as thinking £100 cards are high end.
Let's not try to pretend otherwise. Average "joe" knows absolutely nothing about graphics cards.
Another thing people seem to be struggling to understand is that while it looks backwards on AMD's behalf, they're restructuring their graphics card line, it doesn't seem like this is just a one off and when the 7 series is out, they'll do something completely different with the names. They've seemingly wanted to change the naming scheme around a bit anyway, look at the 4870X2 and then the 5970.
It's clear the intention isn't to mislead people in to buying a card with the same performance, just think about it, if they wanted to change the naming scheme, how do they do it?
A 6870 uses a different GPU to a 5870, it's set in a difference place than the 5870 now. Comparing it to all the revisions the G92 chip went through is misleading because it was clear that was nVidia's intentions. With the rebadge, came a new, higher price.
The amount of confusion of it as well, and then the black listing of sites who called nVidia out on the fact that a GTS250 was a renamed 8800GT. Just think about how many times G92 has been renamed and shoved in to a load of different products, and how the price often went up each time. That's not taking in to account the non-G92 rebrands as well, like the 6 different 9600 named cards.
