My log is full of messages like these. What do they mean?

2010/11/26 11:33:49 [alert] 28730#0: *693848 stalled cache updating, error:0 while closing request, client: IP, server: IP
2010/11/26 11:33:49 [alert] 28730#0: *693853 stalled cache updating, error:0 while closing request, client: IP, server: IP
2010/11/26 11:33:50 [alert] 28730#0: *693861 stalled cache updating, error:0 while closing request, client: IP, server: IP
2010/11/26 11:33:52 [alert] 28730#0: *693887 stalled cache updating, error:0 while closing request, client: IP, server: IP
2010/11/26 11:33:53 [alert] 28730#0: *693914 stalled cache updating, error:0 while closing request, client: IP, server: IP

CentOS-5.5 x86_64

nginx version: nginx/0.8.53
configure arguments: --sbin-path=/usr/local/sbin --with-http_ssl_module

PHP 5.3.3 (fpm-fcgi) (built: Aug 20 2010 17:09:17)