I've found something, not a solution but at least an explanation, and a work-around.
The explanation
As the ant documentation says:
Note that WTP doesn't support IvyDE's workspace resolver. See the details in the page dedicated to WTP integration.
The work-around
I've created a normal java project and a symbolic link (can be done also on window) to the web-project source folder in the normal project. This way ivyDe can resolve in workspace the normal project and all modifications still "propagate" in the web-project.
Not the best way, but at least...hope it may help someone.