Craig,
If you still are needing help with this, I've done this before and can be of some help. Feel free to contact me - my contact info is in my profile.
Here is a high level idea of what you need to do:
http://quickbase.intuit.com/developer/knowledge-base/how-can-i-auto-assign-records-users-my-application-using-formula-i-want-randomly-and-evenly-1. Read that knowledgebase article and follow the instructions. You should end up with a formula user field. You might label it 'Round Robin Assigned To'.
2. Create 2 new fields: the first one is a normal user type field that you might label 'Override Assigned To'. The second field is a formula user field that you might lable 'Assigned To' that has a formula that looks something like this:
if(usertoname([Override Assigined To])="",[Round Robin Assigned To],[Override Assigned To])
Which basically says: look at the assigned to override field and if there isn't any user selected, populate the round robin assigned user, but if there is a user in the override field, show that user here instead. This field, 'Assigned To' would always have a the correct user value, so it is the one you would use in reporting.
Hope that helps!!!
Eric