My "Sennheiser cx300" playing perfectly. I've been let me told the problem is not that people "break" the 3.5mm standard - the problem is that the standard is too lax - for example the expected voltage isn't documented! Joel On Software detailed this while giving an example about problematic standards:

http://www.joelonsoftware.com/items/2008/03/17.html

So yes, not all headsets are compatible with all players, despite having the same 3.5mm connector...