Set computed field to the value of hierachichal taxonomy

Problem Summary
I have a content type (animal) for which there is assigned a vocabulary (habitat) with various terms (africa, asia, etc) each having its own sub-terms (africa -> east africa, north africa; asia -> india, china, etc). Obviously, it would be possible for each node to have multiple terms from this vocabulary (e.g. a DOG has habitats in various regions of AFRICA and ASIA). I would like to also create a field (field_habitat) which contains the list of all assigned taxonomy terms e.g. if the taxonomy terms Africa>East Africa and Asia>China are selected, this field should have the value "Africa | East Asia and Asia | China".

I would really appreciate getting some detailed advice from anyone on how to achieve this with computed fields.

The main difficulty I have is how to access and reference the taxonomy terms given the node ID. The taxonomy terms are (apparently) stored in an array (I can use drupal_set_message('$node=' . print_r($node,true));) to see the array details but I am very new to Drupal and PHP so I don't know the syntax to use.

I have already tried multiple modules with little success and I'll detail some of my efforts here in order to give a sense of why I have chosen not to use some other modules

Content Taxonomy - This module would have been the perfect choice but version 5.x-1.x-dev (07-July 2008) does not work with Hierarchical Select (5.x-3.0-rc1). Drupal gives a configuration error as soon as the module is turned on

CCK Taxonomy - This module has been abandoned and so I do not really want to use it

System Details
Dupal 5.7
PHP 5.2.5
My SQL 5.0.51a

Thanks