AMD won't slotket to ya
That's the initial assessment, at least, according to a couple of posts (here and here) at Ace's Hardware, as well as a Usenet post. The various postings have some interesting information that suggests, for several reasons, the new Socket A processors from AMD won't be able to utilize a "slotket" device to plug into existing Slot A motherboards.
To be honest, I'm not sure if this issue is really that big a deal. Even if an Athlon slotket were possible, it's difficult to say whether one could be made with "Golden Fingers" style multiplier control for overclocking. Since Spitfire is designed to compete with Celerons, if you can't bump up the multiplier, you're probably better off just buying an Athlon.
Even assuming multiplier control could be achieved, while it's possible that Spitfire's full-speed L2 will make it faster than Athlons with external L2 cache, I'm not so sure that the speed of the on-die cache can overcome the huge disparity in L2 cache size (512K vs. 64K). Even if it could, it sounds as though Thunderbird will be out within a month or two of Spitfire, and it will have on-die L2 and plenty of it. Even better, it'll be available in a Slot A package. Let's just hope that package includes a GF connector. Spitfire? We don' need no stinkin' Spitfire! Heh heh.