BTW I happened to catch a "behind the scenes" bit on the tripod scens at the Alias Maya booth at SIGGRAPH... they showed deconstructions of the scenes, and a lot clearer views of the alien machines in isolation (very brief shots tho)...
An interesting note is that the machine that rises out of the water and sinks the ferry boat, is of a different design than the machines that erupt out of the ground and stalk Cruise on land.
Apparently the alien machines were re-designed on the fly to "look cooler" in any given scene, as opposed to being strictly consistent throughout.
-k