My first thought was that a used iPod is worth more than that, and a quick look at eBay shows used iPod auctions at $100 and higher, so $100 seems to be the floor. This means that iPod users can probably get more by selling it to someone else, plus it means that people can't "game the system" and buy a used iPod for cheaper than $100 and get the trade-in (I assume the trade-in doesn't apply for broken units, although this isn't explicity stated in the article).
The article does predict that Dell is unlikely to get many takers.
The iPod is not just about the hardware, it's about the entire experience. Apple still offers the best integrated experience, from using the iTunes music store and application to the (iPod) hardware.The article also points out that if an existing iPod user wanted to change to Dell, it would be no trivial task to transfer his library from the iPod format to the Dell.
Rorschach wrote a very favorable review here in November on the Dell MP3 player and I know others who like them, but I think getting existing iPod users to change will be an uphill battle. I know I'm not interested in trading in my iPod.