net051 My experience does not agree with that at all.
I have a mid 2011 macbook air with a connector like the one in your first picture, and whilst it doesn't disconnect all the time like some have reported, it is also not "safe" -at all-.
Every time anybody has tripped over the cable the computer has been pulled crashing to the floor. This has nothing to do with the strength of the magnet and everything to do with the cretinous design of the later right-angle connector. It is such a good fit into the rectangular recess in the computer case around the connector that a pull on the cable is directly exerted by the metal lug of the cable onto the body of the computer - completely defeating the object of a safely magnetically secured connector. The magnetic force is at 90 degrees to the force of the pull so could never work as a safety device.
The -only- good magsafe I have had on an Air was the original Air connector, with the tapered lug on the cable connector. This always pulled free of the computer before pulling the computer off the desk, as one would expect from the design, none of the subsequent ones work in any useful way at all.
The lower connector I have on my 17" MacBook Pro and my wife's 13" Macbook Pro, where it has always worked as expected.