No one will notice ANY real world difference between the 1.83 to 2.0 model, and the 2.0 to 2.16 model. Even benchmarking software will show very little difference.
Think of it this way: would you have rather waited to buy this just so you can open iTunes 0.0000005 seconds faster?
The only appreciable difference is the size of the hard drive and the higher baseline for memory. 40GB isn't a huge difference, especially considering you can get 500GB external drives for ~$120. The RAM isn't that big of a deal either, since there are always 2GB deals for ~$70 (and it wouldn't matter whether you had 512MB or 1GB in that case, since you'll have to remove both sticks anyway to replace it with 2GB).
If I had just purchased this in the past few days, I suppose I'd be a little bothered by it. However, those of you that are upset because you got it a month ago should not even care. If returning this is any hassle at all (i.e. restocking fee, etc...), then I don't think it's worth it at all.
Apple did this just so they could say they "updated" the MacBook line.