Amazon's German site has listed an "Xbox 360 4 GB Arcade System Bundle", available for pre-order, with a release date of August 20, 2010. It's not specified what form the 4 GB of storage will take, although it is likely to be either a USB stick or internal flash memory, rather than a hard drive as some had hoped (but let's be honest wasn't too likely). For the moment, the price is set at €149 ($190 USD), though Microsoft stated last month the new Arcade unit will be $199 when it hits the US.
Currently, the slimmer version of the 360 is flying off of store shelves; our store is selling out as the shipments come in the door, reminiscent of Wii sales a couple Christmases ago. Check out the link to the listing below.