The following is from WWE.com:
As seen on WWE.com, Unforgiven and Raw this week, tickets for WrestleMania XXV were originally scheduled to go on sale Sept. 20. However, out of concern for the residents of the Gulf Coast of Texas as it pertains to Hurricane Ike, and the recent disaster area declaration made by Texas Governor Rich Perry, WWE has chosen to postpone ticket sales for WrestleMania XXV until Oct. 11, at 10 a.m. CT.
We thank the WWE Universe for their understanding, and send our thoughts out to those affected by the hurricane.