IRS regs § 1.402(g)-1(e) discusses corrective distributions of excess deferrals. A plan may provide that an excess deferral can be distributed to the employee. This can happen either during the tax year or before April 15 of the following year, and slightly different provisions apply in these two cases.
If the plan does not provide for a corrective distribution or for other reasons there is no distribution by April 15, the money must stay in the account. It is still taxable during the year of contribution, though, and it is taxed again when distributed.
IRS Publication 575 is helpful on these matters: on page 18 of the 1998 edition it advises the employee that if he/she does not get a distribution from the excess deferral, he/she must report it on the tax return but the employer does not show it on the W-2.
[Note: This message has been edited by CVCalhoun]