Fusible reports that Microsoft has likely registered all manner of domain names for "Halo Infinity", whatever that is, including .net, .org, .mobi, .co.uk, and .de. HaloInfinity.com, unfortunately, is already registered—it's a Japanese site about curing baldness, or something.
Also registered were HaloInfinityMultiplayer.com, .net, .org, .mobi, .co.uk, and .de. Microsoft is almost certainly the registrant, as the domains were registered through MarkMonitor, through which Microsoft.com and other Microsoft sites are also registered.
Your guess is as good as ours as to what "Halo Infinity" could be (well, ours is probably a little better—we are professionals). Could it be the name for the new trilogy, or a whole new game? As Joystiq points out, "Infinity" is the name of a Halo: Combat Evolved map exclusive to the game's PC port (it was a great, enormous map, for those who remember). What that has to do with this, if anything, is unknown.
Any guesses? Let us know in the comments or on Twitter.