Except for lots of particles hitting the atmosphere every day, showing such high speed collisions are safe from creating strange forms of matter.
Exactly, those particles have much more energy than any we would be able to create there.
The 'strangelet' or the 'mini black hole that would swallow us all' ideas are not sensible scenarios. They arose because, as you know, when you undertake a project of this size you must analise every conceivable risk, no matter how unlikely or apparently ridiculous it is. One of the possibilities that was studied was the creation of a strangelet that would destroy Earth. Needless to say, they ruled out all these ideas
Also, this is not new to the LHC. When the RHIC
WP was being built, a comitee was ordered by its director (John Marburger) to study several disasters:
-Black hole that would eat us.
-Strangelet that would convert all matter on Earth to a new form.
-Triggering of a transition to a new and more stable universe.
If a mini black hole is created, it will evaporate before it can eat anything. As for the other possibilities, your argument is the simplest one: while the LHC will reach very high energies, there are processes going on in our atmosphere at much higher ones and we have not been destroyed yet.
Their conclusion:
"We conclude that there are no credible mechanisms for catastrophic scenarios at RHIC [...] Accordingly, we see no reason to delay RHIC operation."
The page I linked lets you open the full report (27 pages, pdf).
These things always gain a lot of notoriety for well known reasons (abundance of crackpots promoting them + the media loving to scare people).