Right now the added cluster looks like a stop-gap because the 55nm refresh seems to be grievously delayed. This unfortunately does little to help the production cost issue, and really greatly devalues the GTX280 by putting any potential GTX260+ too close to it.
BTW, as for DX10.1 / GDRR5 I think some people need to research the term sarcasm first.
Adding GDDR5 to the GTX260/280 offers very little since they aren't bandwidth limited so much as shader limited, and adding DX10.1 is a complete core-redesign (not a tweak like ATi's HD2K->HD3K) and DX11, it a pipe dream for 2008 for either company.
I can see a GTX260+ coming out, but considering the cost issues, it is more an indication that the rumours of the 55nm refresh being delayed than anything great. The HD4870 trades blows with both the GTX260 and GTX280 and costs significantly less, nVidia needs something quickly to maintain performance, but reduce costs, that this is the back-to-school launch and not the 55nm would be bad, despite probably selling reasonably ok.