That's the question that has been posed quite a bit online lately. (Someone posted this on my Facebook wall, so I decided to check it out).
Aaaaanyway ... I looked into the matter. There's quite a few results for it when searching in google, such as here, however, the main source appears to come from 411mania.com. Their main reasoning for this is that the term "Redguard" is soon to end its trademark status.
This, then, appears to be the sole reasoning behind the claim that this is the new DLC.
Given this fact - I am happy to debunk this myth.
A simple trademark checking website search will reveal that the trademark term has already been de-trademarked. (And has been dead since 2007). In fact, one of the topics on the Bethesda forums (regarding Shadowkey) revolved around trademarks being abandoned.
However, I have been wrong before, so - it may turn out that this is in fact the next DLC. I am merely stating that at this stage, there is no evidence to suggest its existance.
One thing that I think it is DEFINITELY safe to say though: If it is the next DLC, expect PS3 users to still be waiting for it months after its release.
EDIT: Having re-looked at it (apologies, it is very very early morning here). There was indeed one released later here. However, it is identical to the one filed and abandoned in 2007.