I just upgraded to Apache 2.0.48 this evening in Gentoo. Ran into a heck of a problem to debug. Starting Apache from the init script would indicate success, no errors in the log files, but apache would not spawn child procces though it would bind to port 80 and accept connection, just not reply. With lots of searching I came across a forum thread in the Gentoo forums where several people indicated experiencing the same problem on SMP machines. The solution that worked for them was disabling the auth_digest module. In Gentoo, just comment the following line near the top of /etc/apache2/conf/apache2.conf. This problem is likely to exist no matter what distribution you are running and the solution should be similiar. Hope this saves someone else time and headache.

#LoadModule auth_digest_module modules/mod_auth_digest.so