In order for Twitter to start a subscription model, they'd need international SMS (something they seem to have given up on, as third parties now provide paid SMS bundling) AND IM.
Today, I opened my T-Mo statement, and there was a Twitter ad inside. Still no IM and SMS, though. No matter how big their user base gets, that looks like a show stopper to me.
It is to some people, the 'Internet celebrity' (Calacanis etc.) crowd would gladly throw thousands/year at Twitter as it allows them to grow their image/social circle.
Sure they can make some money from subscriptions, but Twitter is not a $50/month or even $50/year service in my opinion.