Command Block

The Command Block is a block made with the intent to support Adventure mode and will allow mapmakers to expand the interaction with the player through server commands. The block can run specified server commands when activated by redstone current. Considering its functionality, they will not be able to be crafted and only administrators will be able to place them.

Usage
The Command Block will execute server commands like giving items or experience to a player.

To change the command executed by the block, simply right-click it, and a GUI will open.

When changing the gamemode, use the command format

/gamemode [survival:creative:adventure] [playername or @p]

So for example, to change the mode to survival for the player who activates the block:

/gamemode survival @p

Disabled commands
As of 12w32a, these commands have been disabled for the Command Block (Please feel free to update probably not complete)