Indeed; these are basically Bitcoin microtransactions. I always thought an "if you really believe this deserves to be ranked higher, invest $0.01 in its prospective well-being" model would be good to base a social news website upon, but real microtransactions have always been halted by the static friction people feel when parting with even a single cent of Real Money. Paying in small amounts of CPU time (that have worth, but don't feel like they do) could get around that.

(Note that, even if this were implemented, you probably wouldn't be doing the work in the browser, but rather just spending (fractions of) coins you had already generated and placed into your bitcoin wallet. Thus, this wouldn't slow down spammers—as they could just buy coins on the open market—it would just make the marginal cost of a single spam transaction more (and hopefully prohibitively) expensive. On the bright side, even if it happened that Bitcoins were too comparatively-devalued to prevent spam, spammers would then, by buying Bitcoins en masse, become the de-facto Bitcoin-to-USD exchange!)

That won't really work. See http://news.ycombinator.com/item?id=2407149 (summary: generating bitcoins takes too long and both client and server would have to trust each other.)

It will work, but the scheme has to be simplified some. Coins can be generated ahead of time, and then the client just pays a 0.001 bitcoin (via browser plugin, automatically) to the server instead of jumping through the captcha hoops. No trust is required, it's just a usual bitcoin payment.

I thought a bitcoin transaction was only considered successful after a sizable number of other bitcoin participants have based their successful computations on it?

If that is correct, bitcoin validation isn't fast enough for instantaneous forum postings when the spammers can be assumed to be as dishonest as fits them.

IIRC, you can almost instantly verify that certain bitcoin wasn't spent someplace else before, which would be good enough. Now, the attacker can use the same bitcoin for several web forms simultaneously, but 1) the scope of this attack is limited by the time window, and 2) if the server discovers a fraudulent transaction in, say, a few minutes, the spammy comment can be automatically deleted then.