This is amazingly good marketing on Apple's part. It fits perfectly in with existing marketing facts.
For example, ever wonder why most things, like fast food meals, come in 3 sizes? Because that's what works best to get people to upgrade. If there are just 2, then people think that it's splurging to upgrade to the larger size. If there are 4, then people think they're getting ripped off. 3 is the most effective number to increase perceived value by the consumer.
This is just like that, instead of having 4 products, 2 of which were almost identical, they now just have 3. And those 3 are well differentiated.
20GB
+10GB AND color
+30GB
Very nicely defined.
Not only that, but the prices are much more reasonable as well. The Base iPod costs $300. Before it cost $50 per 10GB, and $100 for color, to upgrade from that base. Now, those upgrade prices are reduced to $33.3 per 10GB and $16.6 for color. Much more reasonable.
These two things combined will have the net effect that though Apple will not increase full-sized iPod sale volumes much due to this shift--after all, the base price hasn't changed, it's still $300--it will increase ASPs (Average Selling Price) significantly.