I am unsure about if they still cover the warranty if you overclock, but the only current vendor that has lifetime on ATI is Visiontek.
Here is a few tidbits from their site.
The provisions of this warranty shall not apply if, in VisionTek's sole judgment, the product has been subjected to incidental damage including but not limited to the following:
* Damaged while being installed
* Damaged by software or hardware by someone other than VisionTek or by motherboard incompatibility/design
* Damaged by tampering, user error, disaster, misuse, neglect, abuse, power supply, power application, alteration, repair, modification, a fix or replacement by an unauthorized technician that in any way affects its performance or reliability.
I would assume they would not honor the warranty if you overclocked based on that.