Several iPhone users are seeing various battery-related issues. An ongoing thread about it discusses the various problems.
All the problems are not the same, but one common issue appears to be an apparent inability to charge the iPhone to 100%.
Wirelessinfo.com shows photos how two of their three iPhones would not charge to 100%, but a followup from Apple PR suggests that this issue may simply be a software related issue alone, and not representative of any hardware defect:
So, we contacted Apple PR about this, and they told us that "Your battery is fully charged, but the UI (User Interface) is just not correctly reflecting this. We expect to fix this in a software update."
Wirelessinfo.com claims that their early testing of this suggests that this may be true, and they've found that "after an intense series of charging/discharging sessions, the problem batteries on our test iPhones seem to be lasting for longer periods".
In addition, a myth going around is that the iPhone's battery will die after 400 charges, this is not true:
Apple estimates that the iPhone will lose 20 percent of its capacity a darn sight less than 100 percent after 400 full charge and discharge cycles.