Q: Will the new user interface of the 6th generation iPods be available as some sort of upgrade for the 5th generation iPods? if not, there will be no upgrade at all for the 5th gen iPods?
A: Although there are presently no conclusive answers available to this question, based on previous experience the likely answer is no, for several reasons.
Firstly, it is highly likely that the new iPod models are based on significantly different hardware from the previous generation. This makes it impractical for Apple to re-engineer the newer firmware for the now-discontinued models. Further, it may not even be possible to bring some of the higher-performance features like Cover Flow to the older hardware at an acceptable performance level, again due to hardware differences.
Further, the new iPod lineup is a new product, and the new interface design is as much a selling point of the new product line as the capacity increases and other features. The incentive to provide the same software and interface features to older model iPods would naturally discourage people from buying the newer models.
Therefore, the expense required to bring the full firmware feature set to discontinued models combined with the potential impact on sales leaves absolutely no incentive for any company to expend the time and effort required to do this.
That having been said, firmware updates for the recently discontinued iPod models may still become available to address problems and improve compatibility with newer iTunes features, at least over the course of the normal support cycle of these products.