FireLance said:
This may be thinking too much, but one thing that bugged me was how the Autobots and the Decepticons knew that the co-ordinates to the Allspark were imprinted on the elder Witwicky's spectacles. Was it that obvious from the pictures on Ebay and in the Government files?
Both sides apparently had figured out that it was on Earth. There were more Decepticons on Earth at start of the movie (Barricade, Blackout, Scorponok, Frenzy) than there were Autobots (Bumblebee), which suggests that they were the ones who figured it out (probably by tracing Megatron's movements), and the Autobots followed them.
I suspect that there was a lot of scanning and such that took place before the start of the movie, as the Decepticons crept around the planet looking for any sign of the Allspark and/or Megatron. Of course, they didn't find anything.
I also suspect that the Decepticons might've stumbled across a clue somewhere about the military having information on the Allspark and Megatron, and that led them to abandon cover and take direct action. This is after they've exhausted other sources; while they might hold the vermin in contempt, it's still a bad idea to reveal yourself. Note that they were trying really hard to make sure there wasn't any direct proof of their existence; Scorponok was following the SOCOM team, and only attacked when the guy mentioned that he did have a picture of it. That seems to indicate they were willing to let these guys go if they didn't have any concrete evidence of the Decepticons existence, since an attack on them could provide the very evidence they were trying to suppress.
So, in any event, the Decepticons brutally hacked into a classified network that had the information on it. The first attempt (by Blackout) got them confirmation that the vermin knew something, and the second attempt (by Frenzy) got them the name Witwicky. That then led to them finding Sam/Spike*'s eBay auction. A bit of Cybertronian image enhancement later, and voila.
The Autobots' method is not shown, but Prime mentions that they found it on eBay, too. They may have been using different search parameters than the Decepticons, looking for "weird crap" online that might suggest the Allspark, and hitting paydirt when they found Sam's eBay auction. (I also suspect that "they" may have almost entirely been Bumblebee.)
Brad
* - I so want to call the kid Spike, dagnabbit. And his dad should be nicknamed Sparkplug!