The results are in: I asked whether product owner participates in the sprint retrospective. Despite the intensive debate on the scrum development list which led to the poll, the overwhelming majority of respondents - 72% - reported that the product owner participates in all retrospectives as a full participant. Only 13% said the the product owner never participated.
I am also pleased to report that no one invited the product owner but did not let him participate. I recently advised a team which did just that, with the justification he is not part of the team. While there might be some justification for this in the Daily Scrum, I believe getting good and timely responses from the product owner is one of the big challenges in the team. Not letting him talk smells of a power struggle and is not motivating fo rthe P-O in any case - why should he come if he has nothing to say?
The poll is still open and you can download the results directly from doodle. As I write this, there are 32 responses. Although those are enough votes that I have some confidence in the trend, it doesn't exactly qualify as a large sample. So if you haven't voted, please do so! If the the trends reverses, I'll post an update.