The following is an excerpt from page 7 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding the Skirmishers deploying as Scouts.

Many skirmishing units are also Scouts, and can therefore deploy further onto the battlefield. Combined with the special movement rules of skirmishers, these units can work their way towards the enemy deployment zone very quickly. Not only does this threaten war engines and the rear of your enemy's advancing units, this can also stop your foe from marching, causing no end of havoc to a well thought out battle plan.

However, there are restrictions on where Scouts can be deployed. A canny opponent who is facing an army they know might contain Scouts will take this element into consideration. As Scouts have to deploy out of sight of the enemy, most Warhammer battlefields will only have a few places where Scouts can deploy. With regards to the deployment of Scouts, units are assumed to have a line of sight outside their normal arc of 90° to the front. The best way to think of this is to deploy the Scouts in a position where they cannot see the enemy, which by default means that the enemy cannot see them. This is to prevent Scouts ending up in an odd position deep inside the enemy deployment zone just because enemy units happen to be facing the front.

There is also the added restriction that Scouts must be 10" or more from the enemy. This means that if you deploy your army so that potential hiding places for Scouts (inside a wood for example) are within 10" of a unit, you can narrow down their potential deployment areas. This 10" restriction exists to make sure that Scouts can't stop the enemy marching on the first turn - Scouts and skirmishers are supposed to be a thorn in the side, but a restriction to marching right from the start would cross the line into frustration rather than being tactical and entertaining.