I have never read Bart Ehrman, do you have a specific book of his that would shed light on Luke as a historian?
Your Wikipedia article on the "Historical reliability of the Gospels" does nothing to prove or negate any question of Luke as a historian. The article on Quirinius has two paragraph on Luke near the bottom:http://en.wikipedia.org/wiki/Census_of_Quirinius#Historicity_of_Luke.27s_details
Neither of which rule out Luke being correct or incorrect on the matter, and neither of which addressing any of the texts I mentioned in the earlier post.
The article on the reliability of Acts brings up a handful of smaller contradictions of "disputed accuracy"; none of which expound on themselves, none of which give give a good, academic approach to both
sides of the argument, and ultimately, none of which leading one to doubt Luke as a good historian. If Luke would have said that Nero was emperor when Jesus was born, there would be issues.