Actually, you can still get out with a 1 tile lip. When you climb to the top of a ladder, you actually stand on top of the ladder block. Then you just jump the 1 tile gap.
True, but it's kind of awkward.
That, and, if the trapdoor works like the current doors and continues to occupy a thin strip along the side of the block, there's a 50% chance one of the trapdoor panels would lie above the ladder, thereby blocking you when you try to stand on the ladder. (Then again, careful positioning of the trapdoor could prevent this...)
Or the trapdoor could be on the bottom of the tile, like snow, instead of being on the top. Then the ladder could come all the way up.
So long as you can still smoothly walk over it, I'd be fine with this. Still the issue of "what happens to the panels when the door is open?", though, since the hanging panels would probably hang down into the empty cube below.
Either way, I wasn't completely serious about debating the details of a trapdoor implementation (though it would be a cool feature - hatch covers are darn useful things!). I really just pointed this out to show that adding trapdoors
isn't trivial:
A block that on activation goes from solid to nonsolid (and changes it's graphic) is a 5 minute code job.
*ahem*