Well, now you can.
From the press release:
IBM is announcing it is opening up its development platform based on Web 2.0 technologies for developers to collaborate and contribute to software under development at www.Jazz.net. Jazz.net is an open, commercial community designed to help companies globally and transparently collaborate on the development of Jazz-based technology.
Previously only available to IBM customers, academics and partners, Jazz.net is now open to the greater software development community. IBM's vision for community-driven development expands the boundaries of how the open source community can impact global software development by not merely creating a vehicle for feedback after products have been developed, but enabling the global community to be a part of IBM's development process from start to finish.
So, hop on over to Jazz.net and sign up!