Hello Alexis, Daniel and Raphaël,

The module developed by Alexis is a valid option me. Let me expose the two alternatives that I can see as being valid.

First of all, some key definitions of project management terminology as defined by PMI:
- Control Account
A control account is a management control point where scope, budget, actual cost, and schedule are integrated and compared to the earned value for performance measurement (3). 

- Planning Package
A planning package is a work breakdown structure component below the control account with known work content but without detailed schedule activities (3).

- Work Package
Same as a planning package, but with detailed activites defined within.
Work package is a more deliverable oriented definition of the work to be completed (identifies work to be done to complete the project scope, but does not provide the low level detail). 

- Activty
Activities represent the effort needed to complete a work package. Each work package is decomposed into the activities required to produce the work package deliverables (4). Activities more specifically identify the individual low level actions needed to complete the work package. 

- Milestone
Milestones are similar to regular schedule activities, with the same structure and
attributes, but they have zero duration because milestones represent a moment in time (5).


Now let's see how OpenERP seems to be mapping these concepts.
- Control Account --> project.project
- Planning Package --> project.task
- Work Package --> project.task
- Activity --> ?
- Milestone -> ?

From a PMI terminology perspective, OpenERP's project.task is now being used more as a work-package than as an activity (1),(2):
- project.task is now being used to assign work to be done without detailed breakdown (work packages)  from other modules such project.issue or sales order lines that are related to internally procured service items.
- project.task is built with the capability to define multiple stages required to complete the work (analysis, design, ..). That is unnecessary in activities, but useful in work packages.


Now let's see how can we completely map in OpenERP the PMI concepts. I can see two options here:
Option 1 -> Use project.project to implement work packages and project.task to implement activities.
Option 2 -> Use project.task to implement work packages and project.action.item to implement activities.

Option 1 adheres to my current project management modules (https://launchpad.net/eficent-openerp-project-management), but has the following limitations:
1) Project Issue module should create project.project records as workd packages instead of project.task
2) Sale module should create project.project records as work packages instead of project.task.

Option 2 would require a significant redesign of my modules.

Mapping of concepts in Option 1:
Control Account --> project.project
Planning Package --> project.project
Work Package --> project.project
Activity --> project.task
Milestone -> project.task

Mapping of concepts in Option 2:
Control Account --> project.project
Planning Package --> project.task
Work Package --> project.task
Activity --> project.action.item
Milestone -> project.action.item


References:
(1) http://www.passionatepm.com/blog/work-package-versus-activity-pmp-concept-14
(2) http://pmbypm.blogspot.com.es/2013/12/what-is-difference-between-work-package.html
(3) PMBOK 5th ed., chapter '5.4.3.1 Scope Baseline'
(4) PMBOK 5th ed., chapter '6.2 Define Activities'
(5) PMBOK 5th ed., chapter '6.2.3.3 Milestone List'