Ok, so I'm taking a look at the node_access() function calls for a product node. It looks like it's failing when it's calling the hook_access() for the uc_product module. Via some var_dump() calls I see that node_access is calling module_invoke() three times, with the $module and $op parameters and the returned value of module_invoke() as:

"uc_product" "view" returns NULL
"uc_product" "update" returns bool(FALSE)
"uc_product" "delete" returns bool(FALSE)

If I look at the uc_product.module's uc_product_access(), it doesn't have a case statement for 'view', it only has cases for 'create', 'update', and 'delete'. Is this how it is supposed to be or should there be an entry for 'view'? If I do manually set this case to TRUE it does seem to work.