Okay, So this is a speculation on the order of the DLC, Based on info in game files, DLC, and In-game Content...

Dead Money (Reference to future Story-Arc) : NPC speaks about the Last DLC and the events of what happens in it.

Honest Hearts (Involves Joshua Graham, New Canaan, Caesars Legion): Joshua Graham supposedly lives, and you decide to travel with Ringo/Merchant (Depending on the choice in Goodsprings.) to help start a branch in New Canaan, and along the way you meet Joshua Graham (Supposedly dead.) and he would MOST likely want revenge against the Legion, so what I speculate with this is that you either help him take revenge, change his views to accept what the Legion did for the good of the Faction, OR kill him. (The Grand Canyon, Possibly the Great Divide which is involved with the supposed Last DLC.)
Honest Hearts is tied to this DLC from Grafitti in the game, nothing I can tie from a speculation.

Big Empty: It may involve Tibbits Prison/Research Base (Van Buren.) which would be known as "The Big Empty" which is referenced by two NPC's in Dead Money and content from some scrapped ideas from the Cut-game Van Buren, Alot of old Pre-war technology is said to be there, It's possible that it may involve destroying the base, or preserving it.

The Fourth: Unknown, to be updated.

Lonesome Road(?) (Going southwest.): Mojave Outpost is most likely involved with this possible DLC, If it is, judging from the map of the world then the MOST likely area for it to take place is The Hub, you may leave the mojave to work as a Caravan Guard seeing that the Mojave Outpost is a checkpoint for Caravan's, and that the Crimson Caravan is headquartered in the Hub.
Possibly involving Raiders (Old fashioned.), the criminal factions, or the thieves guild.
The name Lonesome Road is tied to this DLC as it seems to be a reference to a Caravan raid, The Courier being attacked with a Caravan, surviving and traveling to Hub so they could track whoever did it while being the lone survivor.

Old World Blues(?): It would be the final DLC with the Courier, Involving Ulysess, The Great Divide (Speculated by many to be the Grand Canyon.), the possible things involved might be involving Boone in which you would have to decide between him, or Ulysses as Permanant companions, or as another scenario, You would have to help Ulysses fight someone (He is supposedly CL, so it may involve attacking the NCR.) or defend whatever it may be he fights against. (Which would mean you fight against him.)
The name is tied with this DLC as Ulysses has the flag of the Old World on his back, and seems to be fighting for what was once America. (Blues being related to the Courier and his struggle perhaps.)

Someone has took note of Zeni-max purchasing the titles for somethings that seem to be related to Dead Money, I updated them to show which DLC I think they are tied to.

Judging from locations in-game, and inclusion of open-ended content, I feel there will be six DLC's.
Thoughts on my reasoning and speculations?

Wha....? Where did find the info for ALL this speculation you have said? Dead Money only reveals a small amount of your future with another courier (Ulysses is strongly hinted although not confirmed) and the Divide. Where are you getting the info about this "Honest Hearts" and the "Lonesome Road" stuff? Show me where you're getting this info from

Some of the discussion here makes it sound as if the DLCs will have to be played in a certain sequence. Isn't one of the prime laws of a DLC a disconnection from other content? That is, there should be minimal or no prerequisites for starting a DLC.
It's strange that Dead Money talks about future events -- it sounds like Obsidian is going to break the DLC law, doesn't it?

There is going to be a storyline in all of the DLC's.

It'll be interesting to see how they manage that. I foresee future butthurt -- people complaining that they cant play DLC 3 because they passed on the first two, for instance. They'll have to be independent to a degree, don't you think?
Fittingly for New Vegas, it's a gamble.

No, I meant that there will be a background storyline, but they are individual.

I had a feeling that was what you meant, but was not sure.
That's the impression I'm getting, too. Almost a parallel main quest -- looking forward to it.

what about the Vault 11 survivor?
also, I think that Raul's dialogue may be a hint to a future DLC just like Veronica's was.