From the minutes of the TAG meeting on November 2nd 2009:
RESOLUTION: to request that the data-* section be removed from the HTML 5 spec
A bit later:
[Sam Ruby]: hmm... not sure I’d heard concerns around data-* before
[Paul Cotton]: right; don’t expect the WG to be familiar with that.
[Tim Berners-Lee]: data-* competes with URI-based designs such as RDFa
[Sam Ruby]: odd... data-* is local to a page... i.e. to be consumed by js on the page, not by crawlers
[Tim Berners-Lee]: but once there’s lots of useful data-* data somewhere, crawlers will want to crawl
[Sam Ruby]: hmm... yes, I can see the inevitability of that. hmm.
(Link to spec added and names expanded.)