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!)