Shinobi said the first shipment was almost completely sold out in its first week. I don't see how that can be considered "tanked". You just can't expect too much from this.
By the way, Shinobi has quoted an interesting survey conducted by SCE regarding PSP Slim.
http://ameblo.jp/sinobi/
Motives for buying PSP Slim
1:65%:Thin, light and small
2:39%:Design and colors
3:38%:Music, movies and other multimedia features
4:27%:Interesting softwares
5:21%:1Seg Tuner
6:19%:It's Sony!
7:14%:Price
8:13%:TV output
The percentage of female buyers of old fat PSP is just 13%, so there is a significant increase in the case of PSP Slim.