I assume, to cry, a lot
They need a new core to compete, flat out, they can't make something based off such a massive die, cut bits off and sell it at £80 and make any profit. THey are supposedly eating money selling the 260 at £160, and essentially they'd have to eat a further £80 and fuse bits of the core. They need a redone core basically which I'm sure they are working on. We'll have to see though, if they were only planning a huge monolithic die like the 8800/280 cards for the next gen on 40nm then they might have been utterly screwed by TSMC screw up with the process(as that huge die on a low yield leaky process simply won't work). AMD's already small die design worked fine on it so they got rather lucky they could make this die so cheap, and make it work on the leaky process. So its possible Nvidia don't have anything that will scale down well enough to be made on the process yet.
Not sure where it leaves the 4830/50/70 either, the 30/50 will be pointless buys as they might well perform worse and cost more. I assume they'll stop making the 4850 ,simply sell them all as 4870's, then the few that don't work sell them as 4830's at a very cheap price rather than throw the cores out. I think IF nvidia get the 275 out in large quantity(which is not in any way certain) it could easily leave room for AMD to put drop the 4890 to around £180-190 inc vat, the 4870 to £130-160 range with 512/1gb versions at the high low end, and the 4770 around £80-110 mark with the higher end being 1gb and overclocked versions.
Trouble for Nvidia is ATi are making decent profit at every price point and their partners all love them, and Nvidia can only compete on price/profit with the 260, which they eat money on and partners hate Nvidia for.