Concentration in Salah is very important as far as I know.
But is it a sin not to concentrate in Salah?
Short Answer: No.
The presence of the heart or concentration in Salah is one of the most important parts of Salah and any other worships, so that worship without it will be spiritless.
Imam Khomeini says:
The presence of the heart is the key to the closure of perfections and a main door leading into actual felicity and in hadiths from the Prophet and ahl-u-albait (A.S.) it is mentioned very much and to less politeness, such importance is given. 
The presence of the heart is one of the conditions of acceptance and perfection of Salah, not its correctness. That is, if anyone concentrate during his/her Salah, he/she has performed more complete worship.
The Prophet (pbuh) says: "Sometimes one second or one third or one quarter of Some Salah's, will be accepted, and some Salah's will be wrapped up like an old gem and will be attacked on its owner, and you will not have your prayer except Salah with presence of your heart. 
 Salah, Imam Khomeini, p. 31, Institute for the regulation and publication of works by Imam Khomeini
 بحارالانوار، علامه مجلسی;، ج 81، ص 260، مؤسسه الوفأ