Convergence is the concept of things approaching one value. For example, airplanes, hand tools and the new Special Infected in L4D2.
The concept behind convergence is that similar problems require similar solutions; there are plenty of historical examples like those mentioned above, however it's as simple as this. Take a dozen people and one problem, wait a week and you;ll have at least 3 ideas that are extemely similar, some times so much so that they are indestinguishable from eachother. This happens more frequently when there is a limited variety of tools or materials to work with.
There has been a lot of commotion on the Steam Forums about how 'The Charger' was a stolen idea from a fan, and now a similar commotion is being raised about "The Spitter".
The first reaction of these folks, including myself, is to think 'Hey, they ripped me off". This is not really the case though.
In reality, when you come right down to it, it is far more likely that there were already similar ideas to this already floating around, these people just happended to be the only ones, or the first ones on that venue to post about it.
This is most likely the case for the new Special Infected in Left 4 Dead 2. Valve most likely foresaw similar solutions to the ones posted, probably before it was ever mentioned on the forums.
The big issue that most people have made is that these ideas were designed to fix problems in L4D, not to be used in L4D2. This is probably the most valid arguement, however with an announced DLC comming before the end of the summer it is not impossible that we will see some of these as DLC.
Thanks for your effort and continued support!