Perhaps this has something to do with it. I don't have an entry for nid==0, but I do have entries starting at 1. However, it looks like the nid's that are my products don't have any entries in this node_access table. Am I right to assume that every node in the node table should have an entry in the node_access table? Interestingly, I've tried creating some other content that is not related to ubercart, and I'm having the same issue with that as well, no entry get created in the node_access table which I guess gives me the "access denied" error.

So I guess this may be more a drupal issue than ubercart.