Recently reference's have been made to the TF500 with an TF500T FCC Label and a TF500 Keyboard. Speculation has been introduced as to what the TF500 will be and what it will run. Popular rumors are a Nexus 10, or a Windows RT device.
The TF500 will likely be a Windows RT device similar to the Infinity (perhaps with a 720p screen). Based on the keyboard compatibility with the TF201 and TF700T the TF500T / TF500 is likely a 10" class tablet with similar dimensions to the existing prime and infinity.
The TF500T will certainly be Tegra 3 based because of the T in the name. Their may be a TF500KL version as well though it has not seen the FCC.
ASUS will not comment yet: http://www.androidpolice.com/2012/08/17/asus-tf500t-tablet-spotted-at-the-fcc-could-it-be-the-new-transformer-prime/