The whole incident gets classified higher than top secret. To hide the location of All Spark, US president Hoover actually decides to build the Hoover dam around the cube, whose huge concrete structure serves to shield the cubes signals. Also, Megatron is shipped to the site, still in frozen state, and reverse-engineering its innards leads to the discovery of many of the technologies of the 20th century such as the microchip.
Fast-forward to the present: Sam is a geeky teenager, trying to save money for his first car. As one of the money-raising plots, he tries to sell his grandfathers memorabilia on EBay, including the glasses with the All Spark location engraved. This does not go unnoticed, and both Decepticons and Autobots take become aware of Sam. Both races frantically try to get a hold of Sam and, through him, of the All Spark cube, with the Autobots gaining an upper-hand when Sam finally buys his first car which turns out to be Bumblebee, one of the Autobots.
A wild chase ensues, including the usual mix of US military, FBI and the Pentagon including the defense secretary that is common to most summer action movies of the last ten years. A dramatic showdown in downtown LA leaves not a lot of cars unharmed, and allows George Lucas ILM to show that they still rule in the area of special effects.