Ne7ven,
Before you generate your object from EF, you need to set a property on your context object so that proxy objects are not created in addition to the POCO object, otherwise you object will not be able to be able to be serialized.
In order to do that, on your ObjectContext instance, set a property called ProxyCreationEnabled to False. Like this:
MyContextEntities.ContextOptions.ProxyCreationEnabled = false;
Hopefully that will do the trick.
Tom Overton