I know http://uptime.netcraft.com keeps some statistics of which sites run which operating systems, but I'm not really sure if they also track different GNU/Linux Distributions, or even if it's feasible to detect their differences.
They report the following for a site I'm involved with:Linux Apache/2.2.3 (Debian) mod_jk/1.2.18 PHP/4.4.4-8+etch6 mod_ssl/2.2.3 OpenSSL/0.9.8c 17-Apr-2009 64.34.164.28 ServerBeach
Tim