1. Toggle IK on either hand
2. Toggle it off again

For no apparent reason, the hand bends in a stupid way, and I have to change it back. This makes IK annoying to use. Why does this happen? How can I make it not happen?

it doesn't (usually) happen on the legs, so why the arms ?

All I can see is a very slight, almost imperceptible shift of the forearm from the wrist, not the hand on the ones I tried - Aiko and Vicky. Does it happen with all your figures? 

This is the figure I'm using. Is there something specifically wrong with it that causes this? how can I fix it ?

http://www.mediafire.com/?yrnmnokgzq0

I installed the SL Avatar figure and tried it. For me, there is no change in the hand when toggling IK for either the left or right arm. Hopefully someone more IK savvy will be able to help. I found a couple of references to hand distortion  with IK. Perhaps something there will help you.

Posing Hands with Inverse Kinematics:
http://artzone.daz3d.com/wiki/doku.php/pub/tutorials/poser/poser-posing04

Unkinking IK:
http://artzone.daz3d.com/wiki/doku.php/pub/tutorials/models/models-gen44

What version of poser are you on? I have v7.0.0.63

maybe it's a bug that was resolved in another version or somesuch..

Also, those pages look quite helpful, thanks. but I actually knew pretty much everything there, having already figured it out by trial and error. IK is so annoying to use :(

I'm not really new to this, been using poser for over a year. Just that the hand movement issue I'm mentioning has finally become too annoying to not at least try to solve it. It's not actually a distortion. The wrist joint simply rotates when I turn IK off on a hand. Exactly how it rotates seems to vary based on something I can't discern, but the most common movement is a 80 degree bend backwards. (80 degrees because I "Use Limits" and that's as far as it can bend back)

My issue is easy enough to work around, at least. I simply set a pose dot while IK is on, and then reapply that after turning it off. But the frustrating thing is that the hand motion occurs in every keyframe for that hand, so the ease of working around it scales inversely with the length and complexity of the animation. And becomes extremely frustrating if I have more than 9 different poses for that hand in the anim.

Hmmm, I use an ancient version so perhaps it's a new bug that has been introduced. I haven't seen any comments on anything like that though. Other than hoping someone else wanders along, I can only suggest contacting SM tech support. If you have the option to 'Lock Hand Parts' you might try that if you haven't already. My guess would be that it's something about the setup of that little figure since you don't mention it happening with the standard Daz or Poser figures? You might try sending an IM to pjz99 who seems to be pretty savvy on rigging and see if he will take a look at it for you. 

I seem to recall from the help files of an old version of Poser the instruction was not to toggle ik's, use them or don't, switching will give weird results. With ik on you are setting specific xyz co-ordinates for the hand and the arm joints are forced to follow them (in fact the software has to guess where to put them, so the position and rotation might not be true to life), if you turn them off, the hand will try to stay in the position set whilst the arm joints will revert to their bend/twist/side to side parameters so the hand may be forced into weird angles. One solution is to break your animation down into parallel .pz3's then cut the renders together in video editing.
Below is a link to a vid I made a while back, parenting the poser mannequin's hand and feet ik's to another figure (D3) Many weird things happened especially as it seems that ik's get really confused by rotations of more than 180 degrees. The simplicity of the mannequin figure made it easier to disguise grotesquely twisted limbs, when I tried to repeat the experiment with a human figure it became impossible.

http://www.youtube.com/watch?v=L0JxiNdqdRI&feature=channel_page