What would it take to port this module to drupal 6.x?

Thanks for asking. It will take the following non-trivial things:
* Migrate the views style plugin to Views 2
* Migrate the multistep form to D6 Form API

Do you have experience with either of those?

kratib,

Can you comment on the Flag module? It seems to offer very similar features and is already updated for D6.

@mitchell: I installed and tried the Flags module. It is indeed close to what VBO does, but with a significant difference. When you create a view and add a Flags Ops field (the one that acts on the nodes in the view) what you get is an action link next to each node. Thus, if you want to bulk act on all the nodes in a view, you'll have to individually click each of them (actually ctrl-click to keep the view open and click the next node). VBO, on the other hand, allows you to select which nodes you'd like to act upon via checkboxes *and then* apply the action once on all of the selected nodes.

Hope that answers your question.

@kratib: I just posted a feature request to Flag module, http://drupal.org/node/285503 , for bulk operations.