I've often wondered why this briefest of Paul's epistles comes last canonically in them. It's not the last chronologically, since 2 Timothy was written just before Paul's death. I think that I know the answer now, by considering vv. 1, 24.
Philemon was a fellowlabourer (v. 1) like Timothy, Titus, and those mentioned in Phile. 24, hence its inclusion next to the "Pastoral Epistles," though it was written earlier, around the same time as Eph.-Col. Accordingly, I think that it's placed exactly where it should be, since it's written to a fellowlabourer like the other pastorals are. Perhaps we should include Philemon as a Pastoral Epistle instead of a Prison Epistle, considering its placement and content.
