Twitter and revenue

There's been a lot of speculation this week about Twitter possibly charging for its service. And I've been guilty of keeping this speculation going by discussing the subject online & offline. The basis of this dialogue has been whether they will possibly charge brands, companies or individuals.
Well, now the subject has been addressed by the folks at Twitter, who have clearly stated:

"Twitter will remain free to use by everyone—individuals, companies, celebrities, etc."

So whilst they have nothing to report just yet, its a useful exercise to ask this Top100 UK websites and the 7th most popular UK social networking site how they are actually goinf to make any money.

Because, surely it HAS to start making money eventually? It can't just eat $20m in funding and keep asking for more... can it? That would be like the 'good old pre-.com bubble days' where nobody actually cared about the revenue model... until the money run out.

If the rumours are true and it turned down a £500m offer from Facebook, then it must have other plans..... right? How about?

• Contextual advertising (e.g. AdSense)
• In-tweet advertising (e.g. Magpie)
• Collecting/processing/selling information? (Perhaps, but doesn't Facebook have more to sell? )
• White-labelling its service to closed-user communities or verticals
• Selling any remaining 'vanity' user accounts (like personalised vehicle registrations) or at least charging for ther transfer
• Offering a paid-for 'premium' service (e.g. to see all your previous tweets, etc.)
• Charge companies for interfacing with its API
• Sell to Google for more than Facebook's offer

Any more ideas?