But I suspect that Twitter is reinforcing the visual image of the bird, it's shape and color, and style, because it knows it can't defend the words twitter or tweet as trademarks. They're becoming part of the language too quickly. Just as Google couldn't defend the word "blogger" as a trademark. It's way too generic. I'm a blogger, even though I don't use any of their software.
Doug Bowman: "There's no longer a need for text, bubbled typefaces, or a lowercase 't' to represent Twitter."