When using straight Unity scripting, the default function is "function OnCollisionEnter (hit:Collision)". There is no need to compare to a tag and by default it will return the collider of anything it hits.
With the CollisionEvent action, you are forced to choose either:
Untagged, which only fires on objects with no tag at all on them.
A specific tag: meaning it will only check for objects with one single specific tag.
Or "none", which doesn't seem to check anything at all and never fires.
By default this action should always return a collider and fire off the send event and only get more specific if the scripter changes the default setup. Either the behavior of "none" needs to be changed to mean "don't check for a specific collider, just always return whatever is hit" or there needs to be an additional tag labeled "All" that checks for all collisions, tagged or not.
The current way "none" is set up makes no sense: if the action will never fire, then what's the point of it being there? If "none" is just a standard field that's on all actions that can't be changed, then an "All" option would be the better default choice, since it's most self-explanatory and most mimics default Unity scripting.
I thought I was going nuts or had screwed something up when I figured out my projectiles had all suddenly stopped working, then I realized I had recently converted them from straight scripting to Playmaker and hadn't really tested much.
Note: MaDDoX make an updated version of this action way back in 2011 which adds the ability to check for multiple tags. I'm surprised this hasn't been integrated into the mainline yet. It still doesn't, however, address the need for a check for "All" colliders.