Hypersmurf
Moderatarrrrh...
Could you explain that?
Sure.
PHB p286: The target can't be forced into an obstacle.
PHB p56: An area power creates an area of effect that hits multiple targets or creates an obstacle.
Wall of Fire, an area power, has neither Hit nor Target entries; it does not, therefore, hit multiple targets. p56 suggests, then, that Wall of Fire creates an obstacle.
And per p286, you can't force someone into an obstacle.
-Hyp.