I for one, would gladly receive one commecial tweet per hour if it meant helping Twitter achieve financial viability - an open letter to twitter: a monetization idea

That's a fair suggestion, and I could see it work. Turning the magpies of the twitter-sphere to help sustaining the overall platform operational cost rather than giving money to the twitter users seems an interesting enough idea.

My concern however remains the same (will Twitter survive?): it seems everyone on the web tries to make money off advertising. Is this the only viable method? Let's think about the following:

Targeted Advertising

Targeted advertising might drive away some of the hardcore twitter fan and the early adopters if not done properly. I can imagine many would put their keyboard down and say 'Twitter, you've changed'... then move on. Twitter's popularity came from its simplicity and 'openness' (at least the impression of it), and making the decision to introduce targeted advertising must not be taken lightly.

Paid Subscription

Let's consider the alternative; paid subscription. Is it really that bad? It will definitely slow down the adoption rate. But it's not like these late adopters stick around anyway (mashable: 60% of twitter users quit within the first month). Maybe paid subscription might reduce its users to those who really benefit and contributes back to twitter. That may not be such a bad thing.  I would pay $1 a month to use the service.

Make it public!

Or consider Google Wave's proposition on its attempt to replace email. Maybe twitter should just be a public domain. Http wouldn't be what it is now if the platform forces its users to consume one advertorial content per hour.