Evidence for this theory:
-Selim is adopted. By that we know that he's not related to his mother. We don't know who he's related to. Did he come from an orphanage, or somewhere else?
-His manner of speech, of course. His is more childish, but with more adult vocabulary and authority in his voice, I can easily imagine him talking like Pride.
-The similarity of his appearance to Pride's silhouette. The shape of his face. Ears that stick out. Even the hint of a shirt collar on Pride could possibly match up with Selim's jacket.
-By living in the Fuhrer's quarters, Selim would have the same access to Father's lair as Bradley. And, well...maybe the door Pride was hiding behind was for Selim's room?
-Pride says Bradley has been living with humans for too long. Pride wouldn't have this problem, because as Selim, he's only been around humans for the amount of time he's been adopted.
-Pride was supposed to be helping Wrath and Gluttony take care of Scar. However, he/she did not turn up. Could that be because Selim's tutor refused to allow him to go outside? (It wouldn't surprise me if Arakawa slipped that scene in as a hint.)
-We also didn't see Pride in Father's lair when all the other homunculi (excluding Sloth) were gathered (Chap. 31). Could this be because Selim was with his mother at the time?
-When Roy threatened to tell Selim about Bradley's true nature, his reply was simply: "Is that a threat? It's no use. That cannot be my weakness." Well of course, if your 'son' is a homunculus himself.
-It would be fitting if the oldest homunculus looks the youngest (Pride), just as the (formerly) youngest looks the oldest (Wrath).
-Wrath and Pride seem to be close. Or at least, Wrath was willing to discuss his personal feelings with Pride, and Pride chose not to tell Father about it. I think it would be interesting if Pride had a sort of mentor role for Wrath...especially if their roles are switched in their human guises.
Evidence against this theory:
-We don't know how long Selim has been adopted. If it were only one or two years, it's possible that no one would notice that he hasn't aged. If it's been longer than that, this theory would be debunked, because someone would have noticed that he hasn't been aging.
-Could a child just walk around unattended in the bad part of the city (referring to when Pride stopped Envy and Gluttony in Chap. 39)? Unless Pride has some teleportation/telepathic powers...
And finally, if for some reason this theory is true, questions to consider:
-Why is Selim/Pride so interested in Edward?
-For what purpose would Pride need to be in that role?
-What part of Selim's role/personality makes him suitable for the sin of Pride?
I'll attempt to answer the second question. From what we've seen/heard of Pride, I think that he/she is the most "proud" of being a homunculus. Being the first born, he/she would hold the strongest belief in the superiority of homunculi, that humans are insects, etc. In Chapter 49, Pride reminded Wrath that there would be no use in being symathetic, because humans thought of them as monsters. Wrath, being the youngest and living among humans his whole life (he even WAS a human before), would be vulnerable to human sympathies, yet he's in a position where he needs to be the most ruthless. Father could have stationed Pride at Wrath's side to take the role of a mentor and prevent this from happening.
The word "pride" was already used by Wrath to refer to his pride of being a homunculus. By which I mean, he uses the word "hokori" ("pride" in Japanese) to refer to the humans' pride in being human, but he actually uses the English word "pride" to refer to the homunculi's pride. Could this be a hint pointing to Pride's influence on Wrath's thinking?
...Well, either way, it makes a hell of a lot more sense than Kimbley. XD;