John Mark Walker is Fannie Mae's Open Source Program Office Director and a long-time open source community, product, and strategy expert. He founded the Open Source Enterprise Network (OSEN) and is an advisor for Glyptodon, Inc.
Follow his adventures and escapades on Twitter at @jm_stuff. For OSEN updates, see @osenetwork.
You can also contact him on LinkedIn.
John Mark is a regular speaker on the Open Source conference circuit and has written a number of articles on "Open Source Business Models Considered Harmful" and "It Was Never About Innovation".
Oh right - that should have said, you can't modify and redistribute without sharing your changes in source form. At least in so far as "distributing" or "conveying" software is defined by the GPL v2 and v3.