In reply to superturbo:
I had a similar problem with my first Dynafit / boot combo and eventually concluded it was the boots, rather than the bindings that were the main problem. I also skied around with the toes locked on and was told they'd release at something like DIN 16 but I never tested that properly. The problem happened with Scarpa Spirit 4s but disappeared when I changed to a Dynafit boot, hence my conclusion that the boots weren't ideally suited to the bindings (which were Vertical STs, I think.)
After changing the boot brand (now using La Sportiva) I've very rarely experienced pre-release and the potential consequences of either the pre-release or locking the toes and then forgetting to return to normal make it worth considering a boot change if nothing else works.
There are a couple of operational points that might be worth looking at. Apologies if this is old hat but someone reading this might find them useful.
The first one is that the holes in your boots where the toe pins engage can be partially blocked with ice. I don't think that this is very common but Dynafit do build in a fix for it. If you look carefully at the toe pins you can see a small groove in each one. The idea is that when you click into the toe, but before engaging the heel pins, you pick up your foot and put it down a couple of times. That causes the pins to rotate relative to the boot and the edges of the grooves grind out any ice. I've also seen some people carry a small proddler made from coat hanger wire to clean out the holes before going downhill.
The second one (and I think this is much more common) is a build-up of compacted snow or ice under the articulated bar that links the posts that hold the front pins. The bar has a fulcrum halfway across that clicks downwards when you push down with your toe and the space underneath it has to be clear for the pins to move to the fully-in position. (Hopefully this is making sense - it's a lot easier with the binding in front of you.) Again, some people make a point of cleaning that space out after skinning but my personal hack is to glue a piece of closed-cell foam in there so there's no room for snow to build up in. The binding just compresses the foam out of the way when you click your toe in.
Hope that helps!