For some manufacturers, yes. The MSI card I referenced is already overclocked and without looking into their warranty, as long as you don't go past that you would be fine.
XFX on the other hand has a double lifetime, overclocker friendly, warranty. One reason I really like XFX.
From their tech support dept:
"Overclocking wont void your warranty, as long as the card isnt physically damaged or modified, and there are no BIOS modifications it will not effect warranty. - Erik"