Bukkit/CraftBukkit — comment on commit ea390cd6ea8b405202faa5fda7b89e605499c553
File:  line 0
By JustinGuy:

This does fix portals, but you can still get above the bedrock and build above 128. Steps to reproduce: trap yourself in sand with no air between you and top bedrock, disconnect and reconnect, and you are on top of the bedrock and can build up to 256 :)