Breaking down the motivation for Rice's statement is a complex task in the wake of Syrian comments disparaging the peace process, the upcoming midterm elections, and Hezbullah Secretary General Hassan Nasrallah's speech earlier today. Support for the tribunal is a key component of holding Syria and Hezbullah accountable. However, US support would delegitimize the Tribunal's indictment by allowing those accused to claim the indictment is a US/Israel conspiracy.
It appears that Rice is pointing out the Syrian violation of Lebanese sovereignty to demonstrate the US' commitment to Lebanon and the Special Tribunal, and to cleave between Hezbullah and its nationalistic rhetoric. Criticizing Hezbullah alone would be a gift to Nasrallah. By taking the more regional focus and supporting Lebanese sovereignty, Rice is achieving all three US policy objectives.
Given that Rice is known here in Washington as a team player, and holds a position which President Obama raised to cabinet-level, the move is likely a deliberate play on the part of the Obama administration. Obama's outreach to Lebanon has been slightly less than that of President Bush, but present nonetheless. Maintaining the stability of Lebanon is certainly an administration priority in the Middle East. This priority is raised by the now crippled Middle East peace talks. And regional inflammation will draw attention away from the talks and heighten the rhetoric of radical actors.
The move today is not likely to have significant ripple effects, but will likely be an effective component of a slow, steady US strategy of support for Lebanese stability but also the progression of the Special Tribunal.