According to Fusible Microsoft have registered a slate of domains through brand protection company MarkMonitor.
These include HaloInfinity.org, HaloInfinity.co.uk, HaloInfinity.mobi, HaloInfinity.de, HaloInfinityMultiplayer.net, HaloInfinityMultiplayer.org, HaloInfinityMultiplayer.co.uk, HaloInfinityMultiplayer.mobi and HaloInfinityMultiplayer.de.
Infinity was a PC only multiplayer map in the original Halo: Combat Evolved so it is possible that map is being revamped for Halo: Combat Evolved Anniversary, even if it seems a tad bit excessive to register a range of domains for that purpose.
Other domains registered by MarkMonitor also include SpartanOps.org and SpartanOps.net.
Remains to be seen what this means, and whether it has anything to do with Halo 4 or future Halo titles.