Chalk this up to a possible oversight on Microsoft's part, but Xbox Live Silver members are currently reporting that they can play online for free. This isn't part of the free Live weekend currently going on in the States; these gamers have currently been playing online for free since Wednesday.
The catch? It's only for select games: games also available for Games for Windows. You see, Microsoft recently removed the distinction between Gold and Silver for Games for Windows, allowing everyone to play for free. This appears to have to have had a flow on effect to games that are also available on the 360. These include Shadowrun, Lost Planet, and Universe at War.
Don't go getting your hopes up for free Live, as I'm sure that once Microsoft catch wind of this (if they haven't already), things will be fixed and go back to normal. If they don't, however, I can see a bunch of people picking up the surprisingly decent Shadowrun so they can play online for free. Cheap asses...