According to several sites this weekend, including E4G.info (linked above), Polyphony Digital has confirmed that a Track Editor will be included in the upcoming release of Gran Turismo 5 for Sony's PS3 console.
To quote: With the recent rumors and videos that have been going around about Gran Turismo 5 having a track editor, we decided to contact the developers to find out if it was true or not. Now, we have a official answer.
They were kind enough to give us a quick response and this is what a representative said:
"Yes it will have a track editor. You will be able to rebuild a track from scratch or edit the tracks that are already put in Gran Turismo 5."