I spoke about it 2 months ago when I asked Michael Smith of Opera about Firebug and what they have to offer. He did tell me that they are going to have something that solves the same problems like firebug within “this year” (2007) - i had no idea it could happen so soon!
Anyways, all you web developers - please download this: Opera and then get this: Opera Developer Console
Of course, it is all quite shaky still (bad UI and all that). But, I am glad, they have come out with it - soon I can get rid of Firefox forever :)