http://www.rockpapershotgun.com/2011/10/05/yay-spacechem-joins-humble-bundle/

Doesnt show up on steam for me yet but it seems SpaceChem is the latest addition to the Frozen Synapse humble bundle, after Trauma was added a few days ago. Always heard great things about this puzzle so this is quite cool.

Remember, you can get the humble bundle here, and if you pay above the average, you also get the first humble bundle for free

chatting to a support dude on the humble bundle site it seems we need to activate our SpaceChem key ourselves on Steam (they email it), it wont be added automatically like Trauma and others did. Just a heads up to check your emails

my e-mail says "it will automatically show up on your Humble Bundle download page and a new Steam key is available for you to redeem in the key section."
 
so just go to your humble bundle page where you got your other steam key and there will be another blank for spacechem.
 
also, paying above average gets you the humble frozenbyte bundle, not the original humble indie bundle.