Yep, unfortunately they don't announce specific dates for Xbox Live Arcade games. Basically, all of the games in that "list of 10" are in the home stretch... They're more or less done, and are going through testing and certification. So the actual release schedule is based at least in part on what order the games are actually done -- that is, testing, certification, etc.
With a packaged game, there's a long process in getting a game out... After the game is done, the disc has to be duplicated, the game has to be packaged, retailers have to know to put aside shelf space, ads have to be placed in magazines and on websites... So that means packaged games need a month or two between when they're finished and when they get on the shelf.
With an Arcade game, since they're distributed on Xbox Live, that extra wait isn't part of the process. When a game is done, it can be on the service within a couple of days. The downside of this comes with setting release date expectations. With a packaged game, the publisher has to know the release month or more ahead of time due to the process. With an Arcade game, we can be in a situation where a game gets the golden certification stamp and is ready to go, and we decide to put it up the following Wednesday.
The upside of this is that when a game is finished, you guys can be playing it two or three days later -- the games don't sit around waiting to be packaged. The downside is that this quick ship ability means that the Arcade folks sometimes aren't sure which titles will be going gold until very close to the ship date, so you don't get the month-early notification that it's coming that you get with packaged games.
As for there being no game this week, hey, Crackdown is out. Who'd have time to play it anyway? :)
