This famous well-beloved section, known as the Pericope Adulterae, has been the subject of not only many sermons but also many theses and dissertations. Some of the newer Bible versions try to place a footnote stating that the segment is not present in the more ancient available manuscripts. A number of commentators have tried to avoid commenting on it. Some regard the style and theme to be Johannine and opine that perhaps it was omitted in some early manuscript by a copyist and the error carried on into later manuscripts. Some think the omission may have been deliberate in order to protect the community from becoming too lenient towards the sin of adultery. 
Internal evidence points out to the originality of the verses as inspired by the Holy Spirit. The theme of the light of life revealed in the grace of Jesus Christ (John 8:12) is consistent with the pronouncement in John 1:17, that the law came through Moses but grace and truth through Jesus Christ. Also, the contrast between the condemning light of the Law of Moses and the saving light of the Grace of Jesus is obvious. To those who walk in this light, there is cleansing by the blood of Jesus (1John 1:7).
The passage is certainly there in a number of manuscripts though it fails to appear in a number of others. However, the inductive nature of research cannot draw conclusions on the basis of what is not available yet, i.e. a more ancient manuscript that has this passage intact. There is still this possibility that this passage got missed in a copying process in certain manuscripts but remained intact in others. The verses possess marks of authenticity, were accepted and quoted by the church fathers, are consistent with sound doctrine, and are spiritually edifying.
1. John David Punch, The Pericope Adulterae: Theories of Insertion & Omission, Doctoral Dissertation submitted to Radboud University Nijmegen, 19 April 2010