Rafe Colburn says that the Open Source community has plenty of experience with mission critical software, and cites Apache, BIND and sendmail as examples. Apache, BIND and sendmail are buggy, insecure, and crash constantly (people don’t notice, because they do better than shrinkwrap Windows software). I have DNS outages daily. Sendmail is the cause of about 400% of all UNIX breakins. How much allowable downtime do you want on the air traffic control networks? Basic telephone networks have regulated maximum downtimes on the order of 2 hours per forty years of operation (too bad my cell phone can’t find the network for hours after I turn it on).