Is it more prevalent in any particular game?
I think I have heard bowling the most, because of the timing involved in letting go of the "b" button.
I can only comment on the one time but I think it was bowling judging from the angle of attack and where he would be standing.
I think you are right that it's related to the 'release' of the ball. As their hand relaxes to let go of the B button it's more likely to open the whole hand and let it slip out, right on the down/upswing.
It's partially my fault because when we play more than 2 players at bowling we hand the controller about (4 controllers will sort this when they come more available) and I didn't always enforce the strap rule, because it takes a few seconds for them to put on. I didn't use it myself but do now to set a better example.
We have an extra controller so for 1 and 2 player games it shouldn't be an issue.
I have given the straps a really good tugging and they should be strong enough to cope with what the kids can throw at it.
The weak part may be the width of the plastic loop on the controller itself, I got the impression if I meant to break it, that would be the first thing to give.
With a bit of luck this will be redesigned for future versions, not that it's vital though.
The people breaking them have to be giving them a hard time if they aren't faulty units in the first place. My old Atari joystick could poke someone's eye out if you didn't pay attention
Ours was a failure to enforce the strap rule rather than a failure of the equipment.
No Wii for me, 2 days.