Hello!
Some time ago I have reported a problem with U2312HM monitor. The backlight was not turning on intermittently on UAC pop-up, resolution change or resume from standby (original thread http://en.community.dell.com/support-forums/peripherals/f/3529/t/19503831.aspx). That time my monitor was replaced by seller.
It was OK for some time. Nowadays my replaced monitor exhibit about the same behaviour: the backlight does not go on after standby. Sometimes I see uneven backlight in the right side of the panel. Turning off/on helps. This time the problem is less reproducible, so I've got little chance show it to technician. Meanwhile the warranty is about to end.