10 minutes early getting on Space Mt on a recent visit and CM wouldn't let us go. We went to the gift shop and bought the kids some pins to kill the time. So it appears that early entry isn't happening. I didn't really expect it, but we were in the area so we thought we'd try. I don't have a problem with that, but I would hope they would give those who are delayed getting back a bit of a grace period.
Not to hijack the thread, but I do wish they had a different system rather than having to go to the attraction, then come back from who knows where in the park at the appointed time. Perhaps a central FP distribution spot, or ? I haven't been to WDW since FP+ so I don't have experience with that, I just know as I age it becomes less important to us to run to an attraction for a FP, then have to walk back again later etc. We try to plan our days walking so as not to tire out so early etc.