Whilst I agree that it is vital to a project's success to assign the "right" project manager to it, I also think it is worth realising that often a Project Manager may not be given the choice of which projects he/she runs with.
Even the most experienced project manager, when faced with a project that exhibits every possible challenge, can fail.
I don't think it is fair to blame the PM entirely, purely based on their level of training, certification or experience. In many organisations PMs are given projects and told to get on with it.
Ideally, we'd all be selective about the projects we took on, but often (particularly for more junior managers) there is no "choice" involved.
You can manage risk, people, deliverables, requirements, cost, quality, scope and schedule as professionally as possible, but at the end of the day, if you've been given an unrealistic project (such as a poor team and no ability to outsource/contract, unfeasible business case, or non-comitted executive sponsor.) you can stand very little chance of "success".
Some times as a PM the best you can do is perform your role as well as possible within the project environment, and be able to prove that the project failed due to measures beyond your control. A horrible way to work, but one that I'm sure we all face sooner or later.
The last thing any of us want to do is focus on covering our asses, but that's a fact of life, when things go wrong, people only feel satisfied when someone or something is to blame.
In most of the organisations I've worked for, there has always been a mindset where the majority of sponsors and stakeholders did not take any responsibility for the project's success, it was all deemed the project managers duty.
I just wanted to speak up for all the PMs out there that might not have years of experience or formal training/certification, but can still make more than adequate project managers, often because of a diverse range of experience and being involved in projects at the "grass roots" level.
The biggest failure with Project Managers that I see regularly is a focus on technology over business benefit, reduction in scope in order to achieve deliverables at the expense of lower priority deliverables that have an impact on the overall benefit delivery, and I think if the PM applies themselves inappropriately - then that's the #1 route to project failure.