Hi Egoblitz and welcome to the RobotShop Community forum.
A “sketch” (in Arduino speak) is a program that will run on the Arduino. The quote you refer to simply explains that you simply need to upload a ready made program, (sketch) into the Arduino microcontroller in order to make immediate use of the Ethernet shield. You can of course modify the program to make it do whatever you want (including controlling servos) since you have full access to the microcontroller. The Ethernet shield, simply provide Ethernet connectivity for you microcontroller and implements the MAC and PHY layers of the communication protocol.
So, in short:
]Yes/:m] ]You have full access to the MCU/:m] ]Yes, most definitelly (see # 4 for refeences/:m] ]You may find further information below: arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1238880301/:m]
Keep us updated with your project, it looks very interesting!