Obviously not a slam dunk but much more likely than Shawn Marion, let me explain.
First and foremost DAL clearly has a pretty short time frame to win a championship with Dirk. Trading the #13 pick to clear space for Howard makes a lot of sense for his last 2-4 good enough to be a top two guy on a title team seasons. Just adding Dwight isn't going to cut it and anyone can see that a Mavs run will likely have to go through two top SFs in Durant and James. Marion is one of the few defensive players that can keep James from dominating without much doubling and other help and his performance in the Mavs championship season was evidence of that. He is also younger than Carter but most importantly the Mavs still own his Bird rights which means resigning him for the 14/15 season to go over the cap is another great benefit to keeping him (if he chooses not to retire).
Can they clear off the requisite cap space without dumping Marion? Absolutely. The only currently locked in contracts for the Mavs this offseason are
Dirk- 22.7 mill
For a total of ~37 million and almost enough space to sign Dwight outright. The Mavs also have qualifying offers for Collison and Beaubois @ ~3.3 million each and OJ Mayo holds a player option for 4.2 million that he is reportedly not going to pick up.
Shipping out Carter + 13th pick and renouncing Collison and Beaubois puts them under the cap enough for Dwight by enough that even if Mayo picked up his option they could make the trade Carter +13 + Cunningham and still pick up Dwight (I believe).
More likely though is that they want to keep Collison- a decent starting PG and ready to contribute and the trade the Mavs will be looking to make is
Carter+Crowder+Cunningham+#13 for cap space + 2nd round picks/future firsts.
Besides being hilarious for the Cavs since Crowder and Cunningham were the picks from the Cleveland/Dallas swap last year this is also a terrific trade for the Cavs. Carter was solid last season enjoying a bit of a renaissance with a good PER and his career highest TS%. He isn't a leader but he has drifted away from his problem child reputation over the past few years and had reasonable (not great) defensive metrics the past two seasons in DAL. He would be a big upgrade over Gee/Miles if he played near last seasons level and a small upgrade more along the lines of his 2010-2012 seasons.