It's on all the news networks, go to CNN or the local cable channel for the live news. The live shots were pretty gruesome (Alot of blood on the ground), and CBS just showed a photo of runners approching the finish as a fireball can be seen literally seconds before it happened
So far, mostly injuries and no deaths (hopefully) reported. No cause known yet.
Update: Some http://deadspin.com/explosions-reported-at-the-boston-marathon-473008941 of the explosion (pre and post) from a Reddit poster (WARNING: Graphic Images)
Update 2: New video shows that the two explosions were a block from each other, 15 seconds apart.