I have the same issue, though with my own custom skins, and the issue is now present in the main client in Linux. The issue is not present in Windows, though I don't know about OS X.
A future Steam beta client (not sure which one) will have a fix for this.
Still not fixed. Using the current beta, skins are not showing up in the settings dropdown.
Today's beta client update should fix this.
Confirmed fixed, thanks!
Nothing extracted yet.
I'm ussing Steam Beta Client and since the April 30 update, it doesn't recognize skins in ~/.local/share/Steam/skins/ folder.
I've tried Metro for Steam (http://metroforsteam.com/) and PixelVision (http://pulseh.deviantart.com/art/PixelVision-Skin-for-Steam-211426153)
I don't have a windows installation to try if this is a problem with both clients or if this is a problem only in linux.
System Info: http://pastebin.com/Uxq5ihHi