My MacBook Air has been down to about 2 hours of battery life and I finally looked into what the problem might be. At the same my friend who had the same issue took her MBA to the Apple Store and the guy there "fixed" hers. He didn't explain what he did but after some digging we figured out that he reset the System Management Controller.
After doing a full backup I reset mine and now the machine says I have 3 hours 40 minutes of battery life available, though I do have wifi turned off, which seems to really eat up the charge.
From the Apple Support Doc:
The System Management Controller is an integrated circuit (computer chip) that is on the logic board of the computer. As the name implies, it is responsible for power management of the computer. It controls backlighting, hard disk spin down, sleep and wake, some charging aspects, trackpad control, and some input/output as it relates to the computer sleeping.On another note I have seen this machine do some weird things with the memory usage. There have been several times when I quit all my apps and the MBA is still using 90% of the memory.
Over time, the settings in the System Management Controller may become unusable, which can result in operational anomalies with the computer. Examples include not turning on, not waking from sleep, not charging the battery, or not recognizing the AC Adapter, among others.