Part of the reason this conversation always devolves into speculation and anecdotes is the lack of hard data. Residency programs don't release their rank list or scoring algorithm for candidates, and so it becomes incredibly difficult to glean whether Joe DO with his 240 was ranked lower than Jane MD with her 230 strictly because of degree. Confounding variables: Jane goes to Howard and Joe goes to PCOM; they are applying to IM; Jane has better LORs but Joe has a few publications. All we see is the resident roster and whether or not they have matched a DO in the past few years. That tells us nothing about the cohort of candidates the program interviewed.
The one bit of data that would be useful to this conversation that we do have is the % of DO matched into each specialty for the past few years. If I had more time I would tally it up -- curious to know if there has been much of an increase. We can also piece together where candidates are being offered interviews...the anesthesia forums have a spreadsheet going with Step scores and # / location of invites.
Programs definitely care about prestige, and their resident roster is one measure of that. The number of IMG and DO, and even which MD programs their residents graduated from all influence the public impression of their competitiveness. A small minority of places would never take even a true rockstar DO, but how many rockstars do we produce? As cliquesh and others have noted, few DO students cross the 230 mark on Step 1, which is just about average. Add to that mediocre research opportunities, sometimes questionable clinical training (see the story by sylvanthus about getting precepted for surgery by a nurse) and less famous LOR writers...and you have a recipe for a ho-hum candidate. That so many DOs have matched at great ACGME programs over the years and excelled speaks to the scrappy nature of a group of people who, for whatever reason, didn't attend a USMD school but decided to go for it anyway.
For my part, anecdotally, I have encountered a number of chief residents along the interview trail who are DO. At not too shabby places like UWash and Cleveland Clinic. But I'm not kidding myself into thinking that I'm being ranked the same as Jane MD who has a mirror image of my application with a different degree.