Is it OK to have ground wire connected to breaker box wall instead of neutral bus?
The panelboard will be bonded to ground, and according to the National Electrical Code (NEC) it must be done so using specific methods.


  250.8 Connection of Grounding and Bonding Equipment.
  (A) Permitted Methods. Grounding conductors and bonding jumpers shall be connected by one of the following means:
  
  (1) Listed pressure connectors
  (2) Terminal bars
  (3) Pressure
  connectors listed as grounding and bonding equipment
  (4) Exothermic
  welding process
  (5) Machine screw-type fasteners that engage not
  less than two threads or are secured with a nut
  (6) Thread-forming
  machine screws that engage not less than two threads in the enclosure
  (7) Connections that are part of a listed assembly
  (8) Other listed
  means  
  
  (B) Methods Not Permitted. Connection devices or fittings that depend solely on solder shall not be used.


If the specific method used in your situation fits one of the methods listed above, then the panelboard is likely grounded properly. The ground and neutral bus bars inside the panel are surely bonded to the panel itself using one of these methods. Which means as long as the panel is connected to the system grounding conductor using an approved method, the system should be properly grounded. 

If you have doubts as to whether or not the installation is correct. Contact your local government and check on any permits that were obtained for the work, and/or request the installation be inspected by the local electrical inspector.