UPDATE Oct. 2011: A white iPod touch is now available
As of this writing, there's no concrete reason to think that there will be a white iPod touch. It's not entirely out of the question--its iOS sibling devices have white versions; Apple regularly makes iPods available in multiple colors--but Apple hasn't made any public comments about such a device, nor have there been rumors about it.
If there is a white iPod touch in the future, don't expect it to come on the current 4th-generation model. Apple generally releases new versions of the iPod touch every September and doesn't tend to introduce updated models between major September releases. Given that, if there is going to be a white iPod touch, it will appear on the 5th generation model--if it appears at all. And that's something you may not want to hold your breath for.
Last Updated: April 29, 2011