MTOM (Message Transmission Optimization Mechanism) is a way to optimize the transmission and marshalling of SOAP messages which include significant amounts of binary data.
Vivek provides a quick update on MTOM support in JAX-WS 2.0 and highlights a couple of important bug fixes; Santiago has done some benchmarking to find the "MTOM Threshold" - the point at which processing of MIME packages offsets avoiding traditional base64 encoding.
As a practical example of how you might use MTOM - Mark has an entry on using JAX-WS 2.0 to access images from Flickr (previously highlighted on The Aquarium)