According to the Official Team Fortress 2 Blog, they are just about done with the new Scout Pack and their design and coding teams have already moved on to the next pack yet to be named. The weapons and achievements are all nailed down, and they just have to finish up the final artwork on them. They like to do the final art as late as possible to ensure that they don't waste any work, which turned out to be a good decision this time around due to the large number of unlockables they tried out as alternatives to the Scattergun. Balancing his replacement weapon has been very tricky due to large threat difference of the Scout between skilled and non-skilled hands. More on that soon.
So when will the scout update be out is the question. According to their blog they've got an update that should be out tomorrow with a bunch of bug & exploit fixes. Later this week they hope to have another update out that changes the way they store your unlockable items. You shouldn't notice much difference, other than if you play on multiple computers they'll now propagate your item choices. You can think of that update as the one that moves your inventory into the Steam Cloud. Once that's out of the way, they'll be able to ship the Scout pack.
Stay tuned and we'll let you know soon as the scout pack is released!