In order to make sure that your Mac does use the Ethernet port in order to connect to the Internet, you may want to create a new location and to only open Ethernet on it. Then, set it up so that your Mac uses DHCP to connect (the most commonly used configuration in hotels).

By plugging your Mac into the hotel's Ethernet port and using this location, you should be able to get a DHCP address. Indeed, if you experience problems with the "Automatic" Mac OS X location, this may simply mean that your computer is picking up another signal (like AirPort) and tries to use it to connect to a network.

Let me know if this helps!

F.J.