The Touch is on a 2 year upgrade cycle so we should see an updated one this year.
The iPod touch was originally on approximately the same 1-year upgrade cycle as the iPhone.
When the original iPhone came out in 2007, the original iPod touch came out a few months later.
When the iPhone 3G came out in 2008, the iPod touch was upgraded a few months later with bluetooth, integrated speaker, a volume rocker, compatibility with the Apple headphone/remote/mic accessory, and an increased clock speed in the same CPU/GPU.
When the iPhone 3GS came out in 2009, the iPod touch was upgraded a few months later with the same updated CPU/GPU cores, the same increase in system RAM.
When the iPhone 4 came out in 2010, the iPod touch was upgraded a few months later with a retina display (different technology so of lower quality), the same new Apple A4 SoC, new integrated camera, and the same addition of FaceTime.
The only anomaly in this established pattern of regular 1-year upgrades happened this past year in 2011, when the iPhone 4S was released without any corresponding upgrade (of any notable consequence) in the iPod touch lineup.