On reflection, I would say that yields probably aren't all that bad on the top end parts. Why? Because of the HUGE availability of 1.2ghz parts.
I just did a quick pricewatch comparison. There are 9 pages of Tbird 1.2s, more than any of the other Athlon processors. They are also priced dirt cheaply. Taken together, I can only see that this means that yields and binsplits must be very good. Taking the # of pages of price entries as the frequency distribution, this is implying that the peak of the curve is right at 1.2ghz, which implies that there should be a considerable number of processors yielding above this, that aren't in the market yet. Sure enough, there are already 2 pages of 1.3ghz processors listed, and the part hasn't even been announced yet!
Of course, the power consumpution/heat issues might skew the curve a bit, at 1.3ghz+, my hunch is that at those speeds the Tbird won't be a neat OEM solution, as they prefer to use rather basic HS+fan combos that likely won't meet the thermal requirements.
Palomino is the wildcard here. Rumors have been all over the map, from only slight power reduction, to significant IPC and freqmax increases, plus lower power requirements. Only time will sort out what's true, I guess..