Thanks, Joe.
Crae, good points. On the other hand, I do think you can get a general idea of how well skinning has done. It's clearly gotten more popular. The raw numbers and popularity of skinning sites has skyrocketed in the past 2 years.
In terms of apps,
You basically have 5 key indicators:
- Icon packages (IconPackager, IconPhile, etc.)
- OS skinners (WindowBlinds, Chroma, eFX, and UXTheme)
- MP3 skinners (Winamp, Media Player, Sonique, Cool Player)
- Shells & Enhancers (Litestep, Aston, Hoverdesk, DesktopX, ObjectBar, NeXTStart, Talisman)
- Skinnable Apps (Trillian, ICQ, SysMetrix)
Now it's true that I didn't cover every app I listed up there. But I covered the most popular one in each category and for the most part, the ones I didn't cover either didn't exist, are statistically on the margins (do we need to know how well Chroma is doing in 2003 to get an idea of the health of skinning) or had similar programs that provide equally good indicators (Litestep, NextStart, DX, ObjectBar).
But I do agree with you that it's not a comprehensive skinning analysis. It's just something I was curious about and looked into and shared my results. 
It's the old 90/10 rule. 90% of the users use 10% of the programs. The other 90% of programs scramble afer that last 10% of the user base.
Even big decliners like DesktopX still got 1400 downloads per day for its top 5 (compared to 51 for ICQ down from 881 in 2001).