Cannot be legitimately obtained. Only available by hex editing the "level.dat" file stored in the world save folder in a survival world (v0.2.0 and later).
Farmland (works just like desktop edition, saturating when near water as well as drying out back to dirt when away from water or when walked on too much; water flows down the sides of it if placed in water in a manner similar to what glass does; PE's autojump does not appear to work when colliding with farmland)
Invisible Bedrock (The block that surrounds the maps. When placed it is possible to see through walls, but it can not be destroyed because of it's lack of a hitbox. When held, the item texture looks like that of a stone-like item)
All items have values above 255, making it easy to separate the Block IDs from the Item IDs.
The item IDs seem to correlate with item IDs from the desktop edition, based on experimentation by DanielMeltzer.[1] This list is based on a list of all item variables in the Item class. More experimentation is needed to confirm that these IDs do match.
Some invalid item IDs (for example, 3000, 3440, or 1938) results in a placeholder item that takes its texture ID from the damage value; more research is needed on this effect.
These are entity IDs found in the getEntityTypeId methods of both the disassembled Android libminecraftpe.so and decrypted iOS minecraftpe binary files.