JMich: You'd be surprised (or not) at how often I've seen criterias been written... Not to mention the forums and datas...
Maighstir: Pluralising an already plural form and ignoring the singular form entirely, I'm not surprised in the least.
Well, I dare think it depends on the language. For instance I think in french we say 'many scenarios' and not 'many scenarii' because the latin word has been kind of adopted and is not considered a Latin word but a new french one. I really have to go check on that right now because I might be completely imagining things.
Edit : I am right but my example is stupid ->
un scénario (fr) = a sceanario
des scénarios (fr) = scenarii
But now the question is : which words have been adopted and which ones have not.