Sometime we can be forgetful, but now we can combat this forgetfulness with a feature called Find my iPhone/iPad?
iOS 5 users can enable this feature by tapping Settings->iCloud and switching onFind My iPhone or Find my iPad, depending on the device.
Now that you have turned on this feature, you can find your iPhone or iPad by downloading the Find iPhone app or by logging into www.icloud.com with your iTunes account. After signing in, your device will be show on a map with the options toSend a Message and/or Sound, Remote Wipe, and Remote Lock. My suggestion would be to Send a Message and Sound to alert people nearby that you are looking for your device. Your device will sound an alarm for about 2 minutes.
I highly recommend turning feature if you own an iPad, iPod Touch, or iPhone.
If you have iOS 4 or earlier, click here for instructions on how to setup Find my iPhone.