I seem to have encountered a really odd problem. I use Domesticat's skinning method for my entire site; I've used that method since pre-wp. I have three WP installations running various subsections of the site (library/story, blog, and skins listing). Every page on the site is skinned. Library and blog both use wp 1.5.2, and I upgraded carefully with all the correct files, etc. No hacks got overwritten.
But my blog no longer obeys the skin cookie. The library obeys the cookie, and skins just fine, when the skin is changed. They do have a different set of plugins, but both installs use my personal theme, which had worked perfectly with the skin method up until 1.5.2. I have tried turning off and out-right removing all the plugins for the blog. I have tried ripping out all the code except the skin code itself from the theme index.php. I tried copying the theme index.php from the library (which works with the skinning) to the blog. I have tried a totally fresh install of wp 1.5.2, with my-hacks and plugins turned off (that was thirty minutes ago, actually).
None of these things makes the blog obey the skin cookie. The conclusion I have come to is that somehow, it can't find the cookie, although it's finding the cookiecheck file, and therefore resets the skin to the default. Is there a setting in WP I'm overlooking that could be causing this?