SCRUM and volunteer developers

SCRUM development process
SCRUM development process

Funambol engineering team uses the SCRUM methodology to develop software. It’s a very interesting method that seems highly compatible with free/libre open source software development habits. It mandates fast release cycles (like the release early/release often mantra), teams that can self-organize. SCRUM also mandates fixed time (2 to 6 weeks) to complete a development cycle (called iteration or sprint). This last part doesn’t seem to be very compatible with contributions by volunteers.

I’ve been looking for other free software projects that use SCRUM internally to understand how they involve external contributors, volunteers, in strictly time constrained release cycles. Pentaho wiki has a very interesting paper on the topic, but I still don’t understand if they have established a process to assign user stories to volunteer contributors.

I wonder if some have tried and failed or nobody has ever tried this at all.