There appears to be a problem with the failure rate of Apple's iPhone AC adapters (part# MA816LL/A).
Rob Parker from the PowerPage podcast reported that his iPhone AC adapter died about a week after bringing it home on 29 June.
Both me and my wife's iPhone AC adapters died within 24 hours of each other. I notice mine was having trouble yesterday when I grabbed my iPhone in the morning only to notice that it hadn't charged overnight. Last night I grabbed my wife's adapter and it was dead too.
I tested both with other iPhone USB cables, iPod USB cables, with and without the iPhone dock and in several known-good power outlets. Nada. Both phones charge fine when connected via the cable to our Macs so I'm pretty confident that it's the bricks.
Rob return his to the Apple store for a replacement, so I'll probably do the same some time this week.