I respect and appreciate Jono Bacon's polite but pointed reference to Canonical's problematic 'contributor agreement'. Canonical impose it for, I believe, all software projects they sponsor, including Upstart, Bazaar, etc.: It's important to realise that this is simply not the sort of licence grant normally implied by that term, as with, e.g., Apache Software Foundation's implementation of that idea.
It's not a contributor agreement at all, really. Rather, it's an outright transfer of copyright ownership, directly and categorically, from the coder to commercial firm Canonical, Ltd. Your work becomes literally their property, to do with exactly as they please, from that point forward, free of charge.
ASF, by contrast, don't insist on owning your work. Additionally, although ASF's Contributor License Agreement does give ASF broad rights, it conditions those rights on ASF never using your property 'in a way that is contrary to the public benefit or inconsistent with [ASF's] nonprofit status and bylaws in effect at the time of the Contribution'. And, as with FSF, ASF's commitment to that effect has credibility earned over a long period, which cannot be said of Canonical, Ltd., even if they made a similar commitment, which they do not.