I've always wondered. On the one side you get the naysayers who don't think Twitter is significantly monetizable at all.
On the other side you've got the people who think the Firehose is the greatest thing since sliced bread and is pretty much a golden ticket.
I don't think I get Twitter - I've never found the service to be at all useful, and I find the 140-char limit quaint and out of touch with how users want to use the service (see: URL shorteners, tweet expanders, Twitter-speak shorthand, etc). However, I am willing to accept that there's a social network I totally don't get that nevertheless rakes in some serious money (like WoW!)
But what exactly is the value of Twitter's data? Unlike Facebook, Twitter doesn't even remotely approach having a representative sample of the population. My parents are on Facebook, my uncles and aunts are on Facebook, along with everyone from my generation - Facebook has meaningful demographic data. What data does Twitter have that isn't almost exclusively limited to "young, urban, tech-savvy"?
As a marketer or data miner, what is so good about Twitter's data that would compel me to pay The Big Bucks(tm) for such a feed? Maybe I'm missing something, but it seems like Twitter's biggest proponents believe that the company can justify a multi-billion dollar valuation by providing glimpses into the thoughts of a tiny sliver of the population at large.