I was attempting to find the command block using the search bar, and even if i typed "co" the block would not show up. I then promptly searched trough every tab in the menu. i could not find the block.
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The only way to obtain the command block in this version is by having cheats enabled and typing /give [Player Name] command_block

This is not a bug, and works as intended:
12w32a, August 9, 2012:
Added the Command Block intended for adventure maps.
It can run server commands when triggered by a redstone signal.
You can use @player or @p as a placeholder for the nearest player.
This block will not be craftable and is currently not available in the creative mode inventory, but can be obtained via the command /give 137
The command block shouldn't be obtainable by every player in creative mode because they could use it to teleport players around and so on.
You can get it by typing /give <Name> command_block
Commands can only be set by opped AND creative players.
Please edit your summary to something more descriptive. "Command Block" in itself is not a bug, you may want to change it to something like "Command block is not in creative mode inventory" if you believe that is the bug.
Not a bug - can not be attained in creative.
not sure if this is a bug(so only server mods on creative server can use), but a workaround is to type /give playername command_block