Fox ordered 13 episodes of "Dollhouse" up front. This is a fairly standard half-season order, and if the show is successful they'd generally pick up the "Back 9" to bring the season up to a standard 22 episodes. Despite this, however, Fox has been known to screw with shows before - "Firefly" for instance filmed 13 episodes, but only 10 were on the air. It's not uncommon for a show to be performing so badly that the network ditches it before it bleeds any more ratings from the time slot.
Just the same, it's been announced that Fox will air all the filmed episodes of Dollhouse more or less regardless of ratings. http://www.dollverse.com/2009/03/all-episodes-of-dollhouse-to-air.html
This is a little confusing, as they're plugging episode #13 as the "Season finale" though it really should only be a mid-season cliffhanger setting up the back 9.
So no word if they're going for an abbreviated season from the outset like "Lost" and "Burn Notice" are doing, or if it's a soft finale in hopes of 9 more episodes and a sophomore season, but as for right now the good news is that we will at least get to see everything that's been filmed on the air.
It's still not looking great for the show's survival, though.