The Department of Justice’s 30-day review period of Google’s $400 million acquisition of supply side platform AdMeld has ended, so the agency has filed for an extension, Google’s Neal Mohan said in a post on the company’s public policy blog.While the the request for more information is considered par for the course for Google given the size of the deal. Plus, given the complexity of the ad exchange and display space, the DOJ was expected to take its time.Publishers contacted by AdAge, who say that they’ve been interviewed by the DOJ about Google’s proposed acquisition, said that the investigators appeared to have a good understanding of the finer points about the digital ad space in general and real-time bidding platforms in particular. However, the feds haven’t taken a really hard look at potential anti-trust issues within the industry since the initial wave of major interactive advertising deals in 2007, a time that saw multi-billion purchases between Google and DoubleClick, which paved the way for the search giant to move into display, and Microsoft’s acquisition of aQuantive.Nevertheless, it’s natural to view this process as a mere formality, as Google patience with the process has never denied it an acquisition yet. Still, Google is a lot bigger than it was even in 2007 and concerns have been raised that owning AdMeld would give Google with a closed pipeline for the buying—through its demand-side platform Invite Media—and selling of display advertising.The question is whether that would constitute an anti-competitive advantage for Google (NSDQ:GOOG - News). Overall, the DOJ will consider the nature of Google’s expansion plans for AdMeld and how it would affect the online ad market. So far, a number of AdMeld competitors, such as The Rubicon Project and PubMatic, say they have seen increased interest in their respective offerings from publishers who are uncertain about the power that Google could be amassing.RTB is still fairly small, but a recent Forrester study commissioned by AdMeld predicted that spending on RTB will rise to $823 million in 2011, a 130 percent increase over 2010. The lag between now and the approval of the deal between Google and AdMeld could give other supply side platforms the chance to build up their business, thereby diminishing the anti-competitive worries.In the meantime, AdMeld doesn’t seem to be suffering either. Last week, NBC (NSDQ:CMCSA - News) Universal announced that it was building a private marketplace that was powered by AdMeld. The network intends to use the private marketplace, which allows it to carefully select the buyers it deals with for its unsold inventory, as a direct line to the major ad agency trading desks, yet without giving up the near-total control and lack of pricing visibility publishers often complain about when it comes to operating within the usual RTB environment.