Transition plan for PHP5 abomination to Drupal
We have done a few sites where we have connected outside systems to Drupal where the data had to be kept in the outside system.  This is what I spend most of my time working with.

When we do this, we typically create a content type to "stub out" the content in the other system.  The content type only contains the node title and a CCK field for the unique identifier in the other system.  Along with this are a lot of hook_nodeapi functions.  For example, the load hook will call out to the remote system and add the data to the node.  You also need to devise a method for getting the outside data into search results.  There are a few methods for this, but those are too lengthy to get into here.

While there are some downsides, we find this works well and allows normal Drupal stuff like comments, tags, etc.