For the reasons mentioned, I think this could be a performance inceasing technique in java/processing as well. It depends on the kind of sketch. Obviously a sketch which does a lot of resource intensive creation of objects (for example a particle based fire simulation) will benefit the most. A possible pitfall could be that you have a pool of objects in memory that you are not/hardly using. It's good to keep in mind, but in my opinion the best optimizations are focused and practical (instead of theoretical). You take your actual applications, find the weak links and optimize those. And perhaps this is one technique to use, depending on the specific application.

A massive particle system is exactly what i'm doing, with larger particles breaking down into further particles based on radius etc. Maybe i'll give it a go...

Object pooling is often seen as poor practice in Java, because general advice is to trust the garbage collector, the Jit engine, and so on. And indeed, they are probably much better in Java than in AS3, being fine tuned for years and used by lot of programmers...
Now, it is worth a try. I recall JBox2D developers reporting good performance increases by reducing the number of generated objects.
See also the http://www.infoq.com/interviews/smith-low-latency interview.