Update: (2011-03-14): So this does not quite fit perfectly into the Attachment Hub, but it is good enough that with a housing for the shaft I'm pretty sure it will work. I'll take more precise measurements and post any updates.
Update (2011-02-18): Arrived no problem from Mending Shed; Parcel Post was quite fast actually. Don't have the KitchenAid here with me right now so will not be able to test at all for a few weeks.
Our KitchenAid stand mixer is probably the fanciest piece of hardware we own other than our car. I've been thinking for a while that there really ought to be a DIY modding community around this appliance; it's a powerful, well built kitchen motor waiting to be exploited (plus planetary gearing if that is helpful). But I haven't found much so far. Here's one nice rotisserie mod I found, and that's about it.
KitchenAid makes a number of official attachments. Some of them attach to the bottom head where the planetary gearing is, while others attach to a port on the front (the "Attachment Hub") that accepts square shafted ("Power Shaft") attachments. I searched around a little and was able to find a part that I'm pretty sure will fit the attachment hub: KitchenAid 9709315, the "clutch shaft". Amazon sells it.
Note that this doesn't include the shaft housing that will allow you to lock it into place; figuring out a way to hack this together will be the first job before anyone can really use this thing for DIY projects. I'm wondering if putting something together in plastic through Ponoko or the like will get the job done. It probably requires some bearings too.
I ordered one from Mending Shed (slightly cheaper than Amazon and will ship Parcel Post), will update with progress.