Hey guys, I just realized we're all overcomplicating this. You don't need to spawn extra solid blocks, then remove them etc.
I've made a terreria clone where you can place ladders, each of them being a 32*32 block. You can place them in a vertical or horizontal series to make long ladders. All you need to do is give the ladder sprite a jump-through behaviour. This will cause the player to walk on the top of the ladder, instead of falling back down once you've climbed to the top. Of course it also allows the player to climb upwards through it. Then all you need to do is add an event to use platform down jump-through behaviour when the player presses down (this allows descent from the top of the ladder). It also solves the jumping from a distance problem. If you fall on the ladder(s) from above you land on it as a solid. Come at it from the side or below, and you climb.
Much simpler to implement this way, and none of the nasty bugs with the previous method. <img src="smileys/smiley1.gif" border="0" align="middle" />
<img src="http://img687.imageshack.us/img687/826/ladderfixed2.jpg" border="0" />
<img src="http://img521.imageshack.us/img521/6100/screenieki.jpg" border="0" />
** updated to include proper animation support, and one bug fix
*all source graphics copyright their respective creators, only used as placeholder