It has been my experience that savvy clients will often post the same jobs and different jobs on more than one website including Elance, oDesk, and other sites. In addition, several aggregate type job sites capture those job postings for display among their site's job postings. ODesk even posts jobs using services on other sites that are not owned by oDesk itself.
The current overall "trend" appears to utilize the power of social media in order to create a synergy throughout the entire web presence. There are new technologies and new sites that leverage social media aggregation to promote job postings and job search.