This week's LeadingAgile SoundNotes features a conversation between myself and Tim Wise on Stretch Goals. I had a student ask about them in class last week and did not have time to get back to it before class ended.
Even if you have formed an opinion about Stretch Goals, I'd encourage you to check this podcast out. I tend to be pretty anti-stretch goals, but during the interview Tim suggested a type of stretch goal that would actually be a very positive thing for a team to commit to.
You can find the podcast here.