Dear Bob ...
What do you do when the elephant in the room is incompetence -- when several of your teammates are incompetent and this is not recognized by your manager? My solution was to leave, but in an organization sense, what do you think?
[ Also on InfoWorld, Bob Lewis offers more suggestions on dealing with less than optimal colleagues in "Having a skill can be hazardous to your career" | Get sage advice on IT careers and management from Bob Lewis in InfoWorld's Advice Line blog and newsletter. ]
Dear Better ...
The answer depends entirely on the quality of your relationship with the manager and your assessment of his/her character. If, in your judgment, the manager is open to the discussion and your relationship is good enough that he/she considers you a confidant, then in a private conversation express your concern that the employees in question need additional training and coaching. This delivers the same message, but where "incompetent" has a connotation of "bad person," "additional training and coaching" deflects any blame to their circumstances.
If the manager is any good, he/she will figure it out.
Otherwise, either leave, as you chose to do, or be grateful your manager has made it so easy for you to shine as an exceptional employee.
Just make sure you take the steps necessary to make your own strong performance clear to your manager, without deliberately undercutting the employees who are already underperforming and without too obviously being a self-promoter.