First off, remember this story is strictly in the realm of rumors and loose talk. But with the latest Bungie news update came a hint that we would see more classic remakes coming soon.
And a further hint was given when a poster on Bungie.net posted a scan of a magazine cover and promptly had his post deleted. Whether or not the above concept art is actually that for a Lockout-remake map in Halo 3 isn't clear, and this is in no way a confirmation of anything.
Having a version of Sidewinder in the next update, it seems unlikely that there would be a second remake in the upcoming Legendary pack. However, xbox.com is listing the third Legendary map as a returning classic that is a small Slayer map, so this rumor may not be too far out-there.
As for the supposed concept shot, it appears to be of a similar structure to the infamous grav lift platform of Lockout. Whether you liked the map or not, there does seem to be a certain amount of correlation between all this.