I suppose there is nothing inherently ideal about the (arbitrary) 140 character limit on tweets. Why not 180, or 280, etc?
Still, my first reaction was, this is .. a bad idea: the 140-character limit is iconic, it's at the core of their value proposition, and Twitter is going to dilute their brand if they abandon it.
I think it's not only that people sometimes feel limited by the 140 characters that matters, it's also all the other times when people don't feel social pressure to write up longer, perhaps more thoughtful messages, that's important here.