We're seeing the exact same setup right now in shares of Puerto Rican banking stock Popular (BPOP - Get Report). Like SIRI, Popular made a significant run coming into early 2013, only to consolidate sideways more recently. For BPOP, resistance comes in at $29, while this stock has support down at $27. Again, the high probability trade is a bet in the direction of this stock's breakout from the rectangle.
Whenever you're looking at any technical price pattern, it's critical to think in terms of buyers and sellers. Triangles, rectangles and other pattern names are a good quick way to explain what's going on in this stock, but they're not the reason it's tradable. Instead, it all comes down to supply and demand for shares.
That resistance line at $29, for example, is a price where there's an excess of supply of shares; in other words, it's a place where sellers have been more eager to take recent gains and sell their shares than buyers have been to buy. That's what makes the breakout above it so significant -- a breakout indicates that buyers are finally strong enough to absorb all of the excess supply above that price level (and the opposite is true of the breakdown below support at $27).