Seems like the new iPod touch 4th generation will be announced on September 7, 2010 in San Fransisco. Engadget reported that they saw an announcement on Bloomberg.
Quoting engadget “This just so happens to be on a Tuesday and meshes perfectly with Apple’s annual Mac plus iPod touch promotion (which ends on the same day).”

Looking forward to price drops for existing iPods and a new iPhone 4 style Retina display iPod 4 Touch, keep checking back…

Apple holding a media event September 7, says Bloomberg