I've thought about this before because I really like the monster formatting in 4E, but much prefer Pathfinder style character creation. It's kind of a pipe dream though, and I've given up on it because of how difficult the conversion is.
The numbers don't really match up for easy conversion, and I'll give an example by comparing the Kobolds from each game. In each system, 4-5 Kobolds would be a standard challenge for a first level party.
The basic Kobold from the first 4E Monster Manual is the Kobold Skirmisher.
These are its basic stats, written in pathfinder terminology.
HP: 27
AC: 15 (Fort +1, Ref +4, Will +3)
Attack: +6, 1d8 damage
Special Abilities: Sneak Attack +1d8, can 5ft-step and move in same turn
Compare these to Pathfinder's Kobold Warrior:
HP: 5
AC: 15 (Fort +2, Ref +1, Will +1)
Attack: +1, 1d6-1 damage.
Special Abilities: Light Sensitivity, Skill Bonuses
The numbers don't really match up - 4E Kobolds have about as many HP as an ogre does in Pathfinder, and hit as hard if they can get Sneak Attack.
There are also a bunch of terms in 4E that don't really translate over to Pathfinder. While the at-will/encounter actions don't really matter because they don't interact with the PCs at all, the various status effects and conditions that 4E monsters deal out don't really map closely to Pathfinder conditions.