Thats not true, with CVS/SVN you can not commit, log or do anything while the server is down.
With git you can still commit to your local tree, push it to another server or send patches.
While it might slow down workflows using github.com for push/pull its not that complicated to temporarily push/pull via another server.
Playing devil's advocate here... but if you were so inclined you could accomplish the same with SVN.
Creating and hosting a repo locally (or offsite) is trivially easy, and though it might not be as friendly or natural as it would be with git on a daily basis, it's not impossible either.
