The only occurrence of ἐπίσκοπος in the undisputed Paulines (cf. 1 Tim 3.2; Tit 1.7). Probably the term refers to a function rather than an official position.
Originally, when I had made this comment, I was thinking along the lines of a highly developed position that is evinced in the Apostolic Fathers (e.g. 1 Cl. 42:4f; I.Ph.l. 10:2; I.Ph. 10:2; Herm. Vis. 3 5:1; Did. 15:1).
In a moment, I will nuance my original statement above. But first, let's look at the noun ἐπίσκοπος. Originally, I translated the last part of 1.1 "...with the bishops and deacons." Maybe the translation of ἐπίσκοπος as "bishop" is a bit misleading here (see NRSV). Perhaps a better rendering for ἐπίσκοπος would be "overseer" is a bit more accurate (see NIV, NAS). Towner, in referring to 1 Timothy 3.2, states the problem well:
It seems preferable to avoid the translation 'bishop' since this term carries so much later ecclesiastical baggage with it (Towner; 244).
In the LXX, the term ἐπίσκοπος seems to place the emphasis on the role or function of the overseer (e.g. Num. 4:16; 31:14; Jda. 9:28; Jdg. 9:28; 2 Ki. 11:15,18; 12:12; 2 Chr. 34:12,17; Neh. 11:9,14,22; 1 Ma. 1:51; Isa. 60:17; see Fee; 68). Moreover, the NT emphasis also seems to share this accent on the function rather than a titular nuance (Acts 20:28; 1 Tim. 3:2; Tit. 1:7; 1 Pet. 2:25).
This point, however, is not to deny that ἐπίσκοπος was a distinguishable position within the community. One needs to look no further than the distinguishing preposition σύν ("with"). In other words, they ("overseers and servants") are singled out from the rest of the community as they are distinguished from "those who are in Philippi." This demarcation would be pointless if the position were not a recognized one in the Philippian community. Therefore my opinion on this verse should have read "Probably the term emphasizes the function rather than the title of recognized members of the community." Or as Fee states: While one need not doubt the titular implications of this usage, the accent is on function (Fee; 68).
The question remains, however, why does Paul single these two groups out? Is it to prepare the way for the rebukes and criticisms that occur in the body of the letter? (Silva; 41; O'Brien 49-50). Fee points the way to Phil 4.2-3, "where Euodia and Syntyche, who are most likely to be reckoned among these leaders, apparently are not in full accord with each other" (Fee; 69).
Whether one agrees with Fee's assessment or not, it seems safe to suggest that Paul has included the leaders in his salutation to remind the community of their place in Christ.
Fee, Gordon D. Paul's Letter to the Philippians. New International Commentary on the New Testament. Grand Rapids: Eerdmans, 1995.
O'Brien, Peter T. The Epistle to the Philippians. New International Greek Testament Commentary. Grand Rapids: Eerdmans, 1991.
Silva, Moises. Philippians. Baker Exegetical Commentary on the New Testament. 2d ed. Grand Rapids: Baker Academic, 2005.
Towner, Philip H. Towner. The Letters to Timothy and Titus. New International Commentary on the New Testament. Grand Rapids: Eerdmans, 2006.