I don't think it's a good idea as it means each @Background could be responsible of the creation of a thread pool. That could cause a performance issue, and it could be hard to understand from a developer point of view. 

I would prefer to define a value. Something like that : 
* [Runtime.availableProcessors()](http://developer.android.com/reference/java/lang/Runtime.html#availableProcessors(\)) * 2
* Math.ceil([Runtime.availableProcessors()](http://developer.android.com/reference/java/lang/Runtime.html#availableProcessors(\)) * 1.5d)

What do you think ?