Our own Joshua Walker has contributed one of the chapters with Andrew Pitts, "The Authorship of Hebrews: A Further Development in the Luke-Paul Relationship." Some time back, Pitts and Walker did an interview (very interesting and worth reading if an argument for Paul's impact on Hebrews is of interest to you, as it should be to everyone) where they gave the basic thesis of their chapter:
The evidence we examine suggests that Hebrews likely represents a Pauline speech, probably originally delivered in a Diaspora synagogue, that Luke documented in some way during their travels together and which Luke later published as an independent speech to be circulated among house churches in the Jewish-Christian Diaspora. From Acts, there already exists a historical context for Luke’s recording or in some way attaining and publishing Paul’s speeches in a narrative context. Luke remains the only person in the early Church whom we know to have published Paul’s teaching (beyond supposed Paulinists) and particularly his speeches. And certainly by the first century we have a well established tradition within Greco-Roman rhetorical and historiographic stenography (speech recording through the use of a system of shorthand) of narrative (speeches incorporated into a running narrative), compilation (multiple speeches collected and edited in a single publication) and independent (the publication of a single speech) speech circulation by stenographers. Since it can be shown (1) that early Christians pursued parallel practices, particularly Luke and Mark, (2) that Hebrews and Luke-Acts share substantial linguistic affinities and (3) that significant theological-literary affinities exist between Hebrews and Paul, we argue that a solid case for Luke’s independent publication of Hebrews as a Pauline speech can be sustained. We don’t claim to have “solved” the problem of authorship in terms of absolutes or certainties, but we do think that this is the direction that the evidence points most clearly.