While there is no evidence at this point to suggest that the shooting was politically motivated, Matt Yglesias points out that an anti-Giffords event was held in June with the billing: "Get on Target for Victory in November. Help remove Gabrielle Giffords from office. Shoot a fully automatic M16 with Jesse Kelly."
Rep. Giffords was also on Sarah Palin's "target list."
As noted earlier, a gun was dropped at a Giffords event in 2009, and her office was vandalized in March.