I recently ran into issues reinstalling apache2 on Leopard version 10.5.4 using MacPorts, and found that the "outdated" command came to my rescue!

I had never used this before, but running it as follows showed me a lot of useful information about the ports on my system:

sudo port outdated

I used the results of this command to cherry pick what I needed to upgrade. Apparently, all I needed to do in my situation was upgrade apr, but I went ahead and upgraded a number of other ports for good measure, following this procedure:

sudo port selfupdate
sudo port install db44 +darwin_8
sudo port outdated
sudo port upgrade apr bzip2 gcc_select libevent libpcap \
libxml2 memcached pcre
sudo port upgrade apache2

After all of this, I restarted apache.

I found http://www.nabble.com/Issue-installing-Apache-2-on-Leopard-td16426260.html to be especially helpful during the process, but mainly was happy to discover the "outdated" command. I hope you find this useful, too.