Wow it is time for this question to have an updated answer! Beginning in 2011, Apple and Samsung have been embroiled in a number of lawsuits and appeals in the U.S., a bunch of foreign countries, and even the civil court in the Hague. In the U.S., Apple alleged that Samsung willfully infringed three of Apple’s utility patents and four design patents, many of which protected user interface elements. The court ultimately found Samsung infringed Apple’s "Bounce-Back Effect”, "On-Screen Navigation", and "Tap To Zoom" utility patents, and design patents that covered iPhone features such as the "home button, rounded corners and tapered edges" and "On-Screen Icons.” In case you were wondering - Apple was awarded $930 million in damages. Importantly, the trial court adopted the standard for patent infringement of user interface elements as whether an ordinary observer would believe both interfaces are the same (as opposed to a stricter “identical” standard).
However, in December 2015, Samsung petitioned to the U.S. Supreme Court wherein it argued that as “the current legal precedent stands, it could diminish innovation, stifle competition, pave the way for design patent troll litigation, and negatively impact the economy and consumers.” The Supreme Court should announce its decision to grant certiorari sometime in 2016.
So to answer your question - search for the patent to avoid infringement! You can start here with Google Patent search. You’ll also want to search the USPTO database. Conclude your search with a simple worldwide search here.