Due to some characters contained in some Text message's the SMS app get's corrupt and fails to start from the SpringBoard. So here's the solution to get it fixed:

p.s. you will loose all your existing SMS's in doing the following procedure, but nevertheless you will have your SMS app working again.

Instructions:
# Connect your phone using iBrickr (Windows) or iNdependece (Mac) or any other File Browser you are comfortable using.
# Browse through the file system and go here: var/root/Library/SMS/
# You will see a file called sms.db delete this file and reboot your phone.

Problem is solved! :-)