Update: Unfortunately, neither ASUS nor NVIDIA had anything of value to say about this device:
Officially, no comment on unannounced products.
First I have heard of it, especially since the next tablets up on the roadmap are Windows based.
As many readers have already pointed out in the comments, perhaps this will be a Windows RT tablet. Time will tell.
This is a curious one; a mysterious ASUS tablet has shown up out of the blue at the FCC today, with the model name 'TF500T'. Naturally, that places it right in the middle of the company's existing tablet line-up, between the TF300 and TF700, but that's all we have to go on for now.