The Greek phrase ο κυριος μου loosely rendered reads "the master of me," and I can agree with H3br3wHamm3r81 that:
"In John 20:28,..., ὁ κύριός μου [is] simply saying what would be equivalent to the English phrase 'my master.'"
Nevertheless, and ever since the days of Monarchianism and Patripassianism (heresies of the 2nd and 3rd centuries CE, respectively), the words ο θεος μου have apparently created problems for both early and modern interpreters. Prof. Ehrman (The Orthodox Corruption of Scripture, 266, 1993, citing Raymond Brown's The Gospel According to John 29, 1966) wrote that one Theodore of Mopsuestia (d. 428 CE) later "argued that Thomas [seemingly ecstatically] uttered those words in praise of God the Father" (i.e., יהוה, the God of first century CE converts from Judaism). And that makes sense to me if, as Strong's G611 and H6030 indicate, Thomas began to give praise to YHVH and was interrupted by Jesus (20:29) commending his (Thomas') belief, misconstrued though it might have been.
Refer back to the Greek of John 1:1 and note that θεος is anarthrous. But in John 20:28, Jesus is specifically called ο θεος (the supreme Divinity, or God; cp. Strong's G2316, et al). So what is the average Bible-reading Christian to think except that Jesus IS "God" himself? And when you combine John 20:28 with 1 John 5:7-8, it's little wonder that Trinitarians believe that Jesus is the God of the OT.
I don't have access to a copy of Codex Ephraemi (C/04, ca. 400-499 CE) but, if I recall correctly, it and other Gospel mss. omit the article, indicating to some interpreters that Jesus of Nazareth, while he might have been θεος (divine-like or godly), is not ο θεος; i.e., THE God Orthodox Christians claim him to be. I also do not have anything by Metzger which addresses John 20:28. Does anyone else know what Metzger said about it?