5 hours is a long delay for that population - I've only experienced delays of a few minutes with larger populations.
The best way to see what happened would be to check the "Results" tab of the campaign. It'll let you know if the emails were sent out or what happened.
Did you send any other emails to these in the 24 hours prior to sending them today and if so, was that activity logged?
~10hrs. following "Send now" punch on campaign schedule 8/8/13 100% of contacts in Campaign remain in "Send Email" state in "Results" pane. My CRM has associated the email as going out to each contact but Marketo has not confirmed the emails have actually been "sent" or "delivered." No change to email activity type has taken place in Marketo via "Results" pane since launch. I also have not received any reply-tos or OOOs, which is further confirmation that the emails have not yet been delivered as scheduled.
Could this be a setup issue? If not, is there anyway to cancel or override the campaign to keep any emails from being sent (ie. so we can create a new campaign to try the blast again without risking sending 2 identical emails to each contact)?
Justin you should open a support ticket on this. It may not be a support issue specifically, but they'll be able to look at specific data related to this and confirm the source of the issue.