FX line is also multiplier unlocked... and even now, the FX-51 isn't that far ahead of the 3400+ on most apps.
True, but if you look at this roadmap from 2 months ago from Anand, the FX-53 (2.4Ghz) and A64 3400+ (2.2Ghz) were supposed to come out at the same time in Q2 04:
A64 3400+ came out early, but the FX-53 did not, so they became misaligned. I don't think FX-51 was supposed to be that close to the top A64 in performance. FX-51 is getting kind of old now.
I think AMD's intention is to keep FX ahead one speed grade in addition to the extra cache, and they are getting it back in line with the FX-55.
I too am surprised by how early it is (originally targeted for Q4), but I think having a part out there that leaves no question as to being the top performer is a good marketing tool, even if they don't sell that many. A64 will benefit from this as well as the fallback parts.
edit: I was a little confused by Anand's older roadmap until I realized there were seperate 3400+ listings in the 754 category. So maybe AMD didn't intend for FX-53 to be out at the same time as the original 3400+. I still think they need the FX line to be a clear leader, and to me that means an additional speed grade.