well, the 5850 is faster on average than the 6850, although they can be pretty close depending on title. they should supply you with something equivelant or better. so i think they are jerking you around. Find some benchmarks and point it out to them that they are replacing your card with something worse and you do not want that.
As others have said the correct replacement would be a 6870 not the 6850 -- Go here and compare the 5850 with either the 6850 or 6870 and you'll see the 6850 is weaker in almost all comparisons where the 6870 is usually equal or sloghtly better which is what an RMA replacement should be (you should not get a worse card as a replacement !)
The 6850 is slightly weaker. Yes it will mostly give you the same experience and will do better in games that use tessellation, but that little extra can make the difference between smooth, and slightly jittery frame rates. I say hold out for another 5850 or a 6870. A 6870 would be a nice little upgrade ^_^
They say that they cannot provide new hd6870 due to the company orders. Sapphire is providing only a new hd6850 or a used repaired hd5850 to me.they think that the hd6870 is above the hd5870 or on par with it.
For hd5850 they said that it might take even a month before i receive my card so i called for the hd6850.