1. Have they mentioned any where when you are required to join them?
2. Have they deferred the joining date in writing or through email?
3. Have they issued any receipt for having received your original certificates?
4. First of all you shall have to understand that now a days joining dates are deferred by almost all companies due to recession and they inform the joining date on receipt of a long term project. It is not that they are deferring the joining dates whimsically,
5. However, your retention bond starts if and after you join the Company. It is not applicable on you if you do not join at all,
6. Your problem is their retention of your original certificates which no body can predict whether they will return it without any hassle or not,
7. However, if they have issued you a receipt for having received those certificates, then you can get those certificates back with legal action, if need be,
8. Talk to the Company asking it to return your certificates as you do not want to join them now & see what they say,
9. I do not think they will object now since you have not yet joined them.