I've been working in critical thinking for the past 10 years or so - both teaching it to undergraduates, and developing better methods and tools for teaching CT.

A couple of quick observations:

First, it is (IMHO) definitively established that it *is* possible to teach critical thinking. That is, you can set up a learning experience such that students demonstrably have much stronger generic, transferable CT skills than they had when they started.

Second, CT skills are acquired in much the same way as any other skills - by extensive deliberate practice. K.A. Ericsson is the relevant authority here, and our approach is simply an application his observations about skill acquisition in other domains to the particular case of critical thinking.

Further reading in this area:

Teaching Critical Thinking: Some Lessons from Cognitive Science http://www.philosophy.unimelb.edu.au/reason/papers/Teaching_CT_Lessons.pdf

Cultivating Expertise in Informal Reasoning: http://www.philosophy.unimelb.edu.au/reason/papers/CJEP_van_Gelder.pdf