The WordPress instance running this site was no longer able to automatically update plugins (and presumably not the core either) after I upgraded from a 32-bit to a sparkling fresh 64-bit PHP install at Joyent. It would start the update, and show a spinning logo and then just hang.
After much debugging, I found out the problem is that the class-pclzip.php that is responsible for unzipping was failing silently with the message:
Downloading update from http://downloads.wordpress.org/plugin/yet-another-related-posts-plugin.3.5.2.zip…
Unpacking the update…
Abort class-pclzip.php : Missing zlib extensions
This isn’t terribly helpful, but digging in, it turns out that class depends on the PHP zlib module, and on 64-bit operating systems (more precisely, operating systems with 64-bit large file support enabled), zlib.h #defines gzopen to be gzopen64. PHP does not protect itself adequately and thus the PHP function gzopen gets renamed gzopen64 as well, this throwing class-pclzip.php for a loop, along with a number of other systems like PEAR.
Automattic should probably patch class-pclzip.php to deal with gzopen/gzopen64 as there are a great many broken PHP installs out there (the PHP bug has been open for over a year and a half without what I would consider an acceptable solution), and it is surprisingly difficult to find a solution online. I guess a great many WordPress installs are still 32-bit, which is kind of sad.