Short answer: revocation of discharge (this is bad).
Nearly every bankruptcy trustee asks you to send to him/her a copy of your tax return. If you do not send the trustee a copy of your tax return then the trustee can, and usually will, motion the Court to revoke the discharge of your debts. You do not want this to happen.
Trustees have to put on a tough face because debtors are always forgetting to hand over their tax refunds. As I mentioned two weeks ago, collecting tax refunds is low hanging fruit for trustees.
Just last week one of the trustees here in Utah implemented a new policy. At the 341 Meeting of Creditors the trustee will schedule a 2004 Examination with you for next year sometime in January to April. This is an extended Meeting of Creditors but the trustee is given great powers to demand that you hand over documents and information. The trustee said that if you get a copy of your tax return before the date you decided on then you will not have to attend the 2004 Examination.
If you filed for chapter 7 bankruptcy then please, PLEASE talk to your bankruptcy attorney before you spend your tax refund.