I also understand Winer's objection to the 140-character limit. I really do. I personally think that it breaks Twitter for me and others who primarily access it via the web or the data connection on our smartphones.
However, I don't think Winer understands that for people who don't access Twitter in this way, anything more that 140 characters completely breaks the service. My wife used to have a "dumb" phone and thus access Twitter entirely through SMS. Had the service broken the 140-character limit, she would not have been able to use it.
I don't know how many people use Twitter in this manner. I don't know if anyone but Twitter does. I suspect, though, that Twitter doesn't change this because they know it will negatively impact enough of their userbase that it isn't worth dealing with.