That scene has made it to a TV commercial recently located online (and the still is at Muppet Wiki), and there is an empty arch where Timmy should be. I would have thought all those monsters were physically there (the arches don't look CGI to me in that shot), unless they just digitally removed one.
Having looked at the still in question, it appears to me that there's supposed to be someone there. Look at The Mutation to the right of Thog and the way he's positioned. Then look at the one to the left and the way he's facing away from Thog. If there was nobody there wouldn't it make more sense for him to be facing towards Thog instead?
I'm guessing that Timmy will be there in the final movie - but represented in CGI rather than a newly built puppet. The shot just wasn't completed in time for the preview. I could be wrong and there's nobody there at all... but it'd be weird given they're obviously trying to recreate the 70s title sequence.
Of course, if Timmy IS included... which version of Timmy will it be? The one who appeared in the titles was an earlier version of him ( the same version that appeared in the dance with Bernadette Peters ) wheras he was later redesigned and had a tail added. Personally, I like later Timmy better...