The next major release of iTunes is currently working its way through Apple's internal testing process, according to recent reports. It seems that Apple has started seeding iTunes 11 to Apple employees for evaluation.
According to a report from 9to5Mac, most of the updates to iTunes 11 will be under the hood. Changes to iTunes's appearance and feature set will be minimal. The main features of the update will be iOS 6 compatibility and improvements to iTunes's integration with iCloud.
The fact that iTunes 11 is built to be compatible with iOS 6 gives a big clue as to when it will be releasing to the public. Major updates to iOS have always been released alongside the new iPhone, and major iTunes releases have tended to follow suit. If iTunes 11 is getting iOS 6 compatibility, then it's a fair bet that it will be launching alongside the next iPhone and iOS 6.