Sahil,
Offer letters do have validity date just to deal with the flexibility fact with joining of candidate. if a company wants to give proper time frame within an employee can join on said position, they can mention the time period after which offer becomes valid. it also helps organisations to final other candidates in waiting list, if they din't receive a confirmation/acceptance of offer within stated time frame.
as far as legal concerns are; I don't think if you have accepted the offer you are binding by the rules and regulations of that company. its the appointment letter makes you bind by the rules and regulations of the company not the offer letter.
Seniors, Pls put guidelines on this..
regards,
Saunee