Umm, anyone who says seiges can\'t be done, you need a lesson in MMORPGs. Not only has it been done, but it had been done fairly succesfully (not perfect). Ever heard of ShadowBane or even DAoC; realm vs realm? I wish people knew what they were talking about before they posted...
Also, in response of the players logging off and stuff, well duh! It is just a factor added to sieges that must be worked around. It takes time, resources, and much planning to get a large number of people to create a siege. Of course people will log off, so what? IceFalcon: Are you trying to tell me you believe all players involved in a siege would have to log off at the same time? That is ridiculous. You have nothing close to a valid point.
Raids are very similar to siege in that you must have a large number of people committed to meet and do a certain task at a certain time. In other words, if you can make a 40 person raid, why not a 40 person siege?