There's already a million of these out there... And I'm not sure how many people will be investing in .US any time soon... I'd say, develop them, as I don't think the domains alone will rake in much cash...
They are both worth one large headache. Both domains contain the word "shack". Radio Shack has a long history of suing everybody they come across with the word shack in their name. They lose more often than they win, but that doesn't seem to deter them. They sued Auto Shack, which chose to change to Auto Zone rather than fight. They sued the owner of www.shackout.com. They even tried to bar the trademark application of Bianca's Smut Shack. They apparently thought that the word shack would cause people to confuse radio with Bianca's Smut.... In the last two cases they lost, but the fact that they did it means there could be monetary implications even if you win. Also note that they have a trademark on shack as well. And that is just the frivolous headache. Imageshack is a registered trademark for an image hosting service. That could be a migraine.