Quote Originally Posted by psstein View Post
It's debated as to whether or not it's a later addition.


It's omitted by Origen in the third century, but Irenaeus makes reference to it in the second.
Right. If it were just one manuscript, I don't think there would be any question. Unfortunately it's a bit more than that. The evidence for omission is still not overwhelming, and many critical translations (e.g. NRSV) include it. Here's what the Word commentary says:

"“Son of God” (νἱοῦ θεοῦ) is missing in א* Θ 28 sypal Irengr,lat1/3 Orgr,lat but present in א1 B D L W pc latt sy co; Irenlat2 (A reads νἱοῦ τοῦ θεοῦ). Though broadly and strongly attested, its omission in several MSS provides basis for question. The internal argument favors the reading (cf. Pesch, 1:74, n. 1; Slomp, BT 28 [1977] 143–50). Not only does the evangelist use the title to introduce Jesus in 1:11 but the Roman centurion’s recognition of Jesus as “Son of God” in 15:39 offers the climactic counterpart of that revelation (cf. 1:11; 5:7; 9:7; 14:61). Although a homoioteleuton at the beginning of a work seems unusual (Slomp, BT 28 [1977] 148), the series of six genitives and the normally abbreviated ΙΤ ΧΤ ΤΤ ΘΤ (Turner, JTS 26 [1925] 150) make this suggestion the best explanation for its absence."

Some commentaries say that this was regarded as the title, and titles were more subject to change than the rest of the text. Of course the question not theologically significant, since "son of God" is used elsewhere in Mark as a characteristic title for Jesus.