Android tablets are set to take over the iPad by 2015, says a new report from IDC. Apple’s iPad may be today’s current leader, but the sheer number of Android tablet-makers means Google’s OS will eventually win out. Following a pattern similar to the smartphone market, Apple’s tablet shares are slowly declining as lower-priced competitors like the Amazon Kindle Fire flood the scene.

The rivalry lives on

Apple shipped 15.4 million iPads in the fourth quarter of 2011, which is up from 11.1 million in the third quarter. But the iPad’s worldwide market share has dropped to 54.7 percent in Q4, down from 61.5 percent in Q3. This drop is largely due to Android’s rising tablet share, with the Kindle Fire leading the charge. Amazon shipped 4.7 million units last year, taking second place with 16.8 percent of the global market. This boosted Android’s market share to 44.6 percent in Q4, up from 32.3 percent in the third quarter.

Rival Barnes & Noble actually lost market share from Q3 to Q4, with the Nook’s presence slipping from 4.5 percent to 3.5 percent. But Samsung’s success has also helped Android make gains in the tablet market, growing from 5.5 percent in Q3 to 5.8 percent in Q4.

One thing that’s still missing from Apple’s tablet is Siri, the voice-activated personal assistant revealed with the iPhone 4S launch. The AI system has become a point of comparison between iOS and Android, though a recent demonstration showed that Android’s voice commands work better than Siri in Japanese and English. As hardware becomes less distinctive across the mobile landscape, perks like integrated personal assistants will become more appealing for consumers, and this is a grand opportunity for Android in the long term.

Back in court

But Android’s battles go far beyond virtual PAs, as Google faces antitrust probes and ongoing lawsuits from its closest competitors. As part of the broader antitrust investigation of Google that started last year, the FTC has subpoenaed Apple to determine whether Google unfairly increases ad rates for competitors, and ranking search results to favor its own services like Google+. As the default search engine for the iPhone, iPod Touch and iPad, the FTC digs into the Apple-Google partnership to see if Android’s OS is being leveraged to control the competition.

In another case, Google’s trial against Oracle has been scheduled for April 16, and is expected to last about eight weeks. Taking place in a San Francisco federal court, Oracle’s lawsuit against Google alleges that Android’s OS infringes on their Java patents. A lawsuit first brought to light in 2010, both Oracle and Google have had missteps in this case, leaving Oracle on shaky ground after most the patents were withdrawn or knocked down after examination from the U.S. Patent Office. The focus of the upcoming trial is expected to circle copyright infringement claims Oracle’s brought against Google, with an ultimate goal to bring Google back into the Java fold, says patent blogger Florian Muller.