Drunsy

I put the iron blocks as a ruler, no mobs spawn inside the area (see the noblocks attach), while adding any block inside will (see the blocks attach)

from what i can tell.. currently the spawner works like this (see the range attach):
+ spawns inside any valid block (light level around is ignored) in a 9x9x3 area with the spawner in the center and middle row
instead of been:
+ spawns only on empty (air) blocks at certain light level in a 7x7x3 area with the spawner in the center and bottom

EDIT: after reading Kahless comment light level does affect the spawner, so clearing the whole area won't work if it remains dark enough.

Note: it still respects valid blocks to spawn inside them but may appear it doesn't since it will still spawn in the block above them if it's valid or air, since doesn't detect the height or "free room" for the mob.

tested again on the 14w26b

light levels.. ignores them completely in all directions..

spawn range... spawns mobs in a range of +4 blocks in x,y directions, and 1 up and down in z

if you make the room wide and lower than that.. won't spawn anything, it NEEDS a block in the mentioned range, even if there is a floor of lava 2 blocks lower

i tested EVERY block in the pic i upload (2014-06-25_21.50.00.png), the space inside the iron block structure will trigger the spawn.