Back in December 2012 we posted on news regarding not just the upcoming 0.5.1 update for Minecraft: Pocket Edition but also the future 0.6.0 update and what both of these updates may very well come with. Well we have confirmation that Signs will definitely be arriving in 0.5.1 but also a few other goodies are on the way.
Everyone can consider 0.5.1 to be a minor version update that will bring mostly bug fixes and optimizations that are needed, mostly for the upcoming 0.6.0 update which should be massive. However, Sign will indeed be arriving in the 0.5.1 update and now it looks like Clouds and bluer skies will also be added to the game. Clouds will be available for fast devices while bluer skies will be for all Android devices.
Gravity will now be applied to blocks like sand and gravel as well. Right now digging through a mountain of sand is safe because the sand won't fall on you. That will change in the 0.5.1 update when sand and gravel blocks will follow the rules of gravity and actually fall now.
There are a few other features being worked on that could make it into the 0.5.1 update which includes Stone Brick Slabs and Stairs. Redstone and Redstone powered objects are also in the works and could make it to the 0.5.1 but I would suggest looking for this in the 0.6.0 at least. Redstone and Redstone power is a big feature that probably takes a lot of work to implement.
Lastly, it looks like Caves are being worked on right now. Apparently Caves are causing a huge drop in FPS right now and may be added to the game as an option that players can turn on or off. That way if you have a high-end device, you can enable caves and not get hit with the performance drop as bad as an older device would.
This is all on top of everything else we posted on back in December of last year regarding the 0.6.0 update which includes things like Armor and even online multiplayer could make it to the 0.6.0 update.