Thank you sir I also had the same question but did not get a proper reply but you gave a complete answer, thanks again.Yes. You can.
The eligibility requirement for Step 2 CK or Step 2 CS are exactly the same as for that for Step 1. Which means you only need to have finished two years in your medical school (finished the basic sciences courses).
Here I quote for you two paragraphs from the current 2011 ECFMG Booklet;
"To be eligible for Step 1, Step 2 CK, and Step 2 CS, you must have completed at least two years of medical school. This eligibility requirement means that you must have completed the basic medical science component of the medical school curriculum by the beginning of your eligibility period.
Although you may apply for and take the examinations after completing the basic medical science component of your medical school curriculum, it is recommended that you complete your core clinical clerkships, including actual patient contact, before taking Step 2 CK and Step 2 CS."
So they are recommending to you that you postpone your Step 2 exams to until after your clinical rotations but they are not saying that you cannot take them.