php-pear currently bundles a bunch of PEAR packages which don't superficially appear to be "core" to PEAR. Packages bundled up in php-pear can't be upgraded (in a nice RPM way) without rebuilding the whole bundle. This of course also means that there is more stuff to distribute (a whole bundle) in the event of a bug in one single package. I would suggest that php-pear includes the absolute minimum of bundled PEAR packages, and all other PEAR modules are packaged separately, so as not to unnecessarily inhibit the upgrading of individual PEAR modules. The minimal set installed by the PEAR bootstrap on http://go-pear.org/ seems to be: PEAR Archive_Tar XML_RPC Console_Getopt OS_Guess which is considerably smaller than the set of packages that the current php-pear bundle provides (which currently includes Net_SMTP, Net_Socket, Net_UserAgent_Detect, HTTP, HTML_Template_IT, DB and XML_Parser in addition to the above)
Done in php-pear-1.4.5-2. I couldn't find OS_Guess so I guess it's legacy.
I had a look into it and it seems OS_Guess is not a "real" package, just a script which is internal to PEAR but named using the same conventions as normal packages. Unlike the other bundled packages, it's not available separately on pear.php.net.