Credits: Robotic Red
I found this on another forum, and thought it was interesting enough to share with you...
Granted, the teeth are offsetting, but it's possible that they're a result of whatever experimentation Orochimaru did on him. Not only that, but the reference to the Hidden Mist Village is a bit telling.
It'd be interesting, as the link with Danzo hasn't been expounded on, nor has Sai's past been built on yet outside of his little picture book.
EDIT: Some more thoughts.
This would provide a very interesting parallel. Sai has constantly been said to have a resemblance to Sasuke. If Suigetsu turns out to be Sai's "brother", then Sai's comments about he and Naruto are alike will have even greater meaning. The pair of Sai and Suigetsu will be parallel to the pair of Sasuke and Naruto.
Also, considering Suigetsu's ties to the Hidden Mist (and he obviously had some direct contact with Zabuza), there are a few ideas to consider.
1) Suigetsu either defected or was taken to Konoha by Danzo before he could finish training with Zabuza. The actual demise of Sai's brother within Roots has not been clarified, so it's possible he may have had some difficulties with his kekkei genkai (turning into water; unclarified), so he "dropped out". And Orochimaru picked him up and took him to do whatever (it might explain the close link between Danzo and Orochimaru...perhaps Danzo gave Suigetsu to Orochimaru in exchange for something?). This doesn't sound plausible, considering how reverent Suigetsu seems to be of Zabuza and the Seven Swordsmen.
2) Suigetsu actually defected from Roots to the Hidden Mist Village, where he underwent training with Zabuza. Then he ended up with Orochimaru somehow; perhaps during Zabuza's failed coup, he got separated and Orochimaru picked him up. This would explain his apparent disgust with Orochimaru.
3) This is probably the most plausible one. During Zabuza's aforementioned coup, Suigetsu got separated from his mentor and ended up in the hands of Danzo. He added to Danzo's knowledge of the Hidden Mist's training regiment (seeing as how Kakashi knew of it way back in the Wave Country arc, it's logical to assume Danzo would also have some info about it), and he paired him up with Sai. Some time later, Suigetsu attempted to flee...or his kekkei genkai(?) acted against when he went too long without water, and he "died of an illness". He's placed into the medical ward before he can be properly disposed of (after all, he'd technically be a missing-nin in this scenario; he'd have to be returned to the Hidden Mist), where a certain medical ninja - Yakushi Kabuto - arranges to have the body picked up by a Oto-nin spy and delivered to Orochimaru for his experiments.
EDIT #2: About Chapter 348.
His comments to the prisoners about how "Sasuke being savior of the world" seem odd. Not quite sure why...but is he trying to get word out about Orochimaru's downfall, in the hopes someone in particular might find out? This is purely speculation, but it'll be neat to find out the outcome of this once news of Orochimaru's downfall reaches Konoha.