We've had trouble finding any information about this, too. But we prevailed!
For the past three years, the Fremont Street Experience has hosted one of the regional qualifying events for the Ultimate Elvis Tribute Artist Contest, the final of which takes place in Memphis each year during Elvis Week (which celebrates its 35th anniversary August 10-18, 2013). The international contest has been taking place since 2007, as far as we can tell, but Las Vegas first became one of the host cities back in 2010, initially on the First Street Stage, and then, for the past two years, on the Third Street Stage.
While the official Elvis.com website currently lists 11 preliminary-event locations for the 2013 contest, including venues in the UK, Brazil, Hawaii, Australia, and Canada, in addition to various U.S. cities, Las Vegas is not listed among them. However, we got hold of the Marketing Manager for the Fremont Street Experience, who responded that the event is tentatively set to return to downtown from May 10-11 next year.
As it's still only "tentative" and not listed on the official site, we'd suggest exercising caution before locking in any dates, if this event is crucial to your Las Vegas experience, although we understand the desire to secure flights as far in advance as possible, before fares go up, potentially.
One related matter, which may or may not have any impact, is the fact that Legends in Concert, the tribute-artist show that performs in several cities, and on the Norwegian Epic cruise line, will be departing its current home at Harrah's Las Vegas this Dec. 30. The Legends brand hosts some of the regional qualifires, including the one confirmed for Hawaii, and in the past has hosted the Las Vegas rounds. The opportunity to sign a performance contract with the show has traditionally been part of the Grand-Prize package, which also includes a $20,000 cash prize, and other perks.
It's not yet been announced whether Legends in Concert will find a new Las Vegas home, or if the absence of this affiliate will make any difference to Fremont Street hosting a preliminary round or not, but we'd advise you to keep on eye on the official website for updates and as soon as we hear confirmation one way or the other, we'll post it in "Today's News" and add an "Update" to this answer.