I just upgraded as well and had the same thing happen to me. However, in my case I am still connected to an exchange server. I know our IT guys are very open with not locking down our phones with stuff like this and I have had Windows Mobile phones connected to the same exchange server with no need for a pin number.
- Open Phone Settings
- Open Location and Security Settings
- Go to Select device administrators
Mine currently says "Corporate Sync" is one of the device administrators, but doesn't allow me to remove it. You may be able to remove it since you have removed the connection to the server.
I suspect either the phone wasn't enforcing the settings set by the exchange server correctly or that there has been a bug introduced in the new version.
NOTE: I know this wont work for the person asking the question, but might be helpful to others. I was able to remove my exchange account, change my password back to Pattern instead of PIN, re-add my exchange account, and the phone did not force a PIN to be set.