If using Xcode 4.2 or higher, try the following:
Click your Project name (in the left column), followed by the Target:
Click the 'Build Settings' tab (in the right column):
Click the 'Release' or 'Distribution' row under 'Architectures', and choose 'Other...':
Double click the highlighted row named '$(ARCHS_STANDARD_32_BIT)' in the popover that appears, and replace it by typing 'armv6'. Then add a new row with the plus button in the bottom left of the popover, and type 'armv7', then click Done:
Update: you should add armv7s to target the iPhone 5 as well. (And drop armv6 if building with Xcode 4.5 or higher, which no longer supports armv6.)
That's it. You should now be able to build/archive without generating errors.
If it still doesn't work, see this answer from justinxreese, which suggests adding entries for armv6 and armv7 under "Required Device Capabilities" in your info.plist file.