hook for taxonomy terms update

Hi,
I use a multiselect hierarchical vocabulary in order to associate users with country/state selection, through the content profile module.
Until now, in my custom module I used the hook mymodule_taxonomy in order to perform custom actions while updating the current terms (country) selection.

I have seen that once I use the hierarchical select with the current vocabulary, hook_taxonomy is no more available, due to the fact that hierarchical_select overwrites taxonomy module, if I have understood well.

I tried mymodule_form_alter in order to achieve the same hook while updating terms for the current vocabulary, with both hierarchical_select enabled/disabled , but the terms passed are the ones before submmitting, not the new ones selected.

I know I am clearly not fond with forms API and module writing, therefore I would like to ask in which way I can implement the equivalent of hook_taxonomy with hierarchical select enabled for this vocabulary?

Thanks in advance.