Some people were already doing this before, but with other formats. I did a little comparison between this and the one Best Buy currently uses, GoodRelations.
Well, the two gists don't quite encode the same information. In the schema.org example, one would still have to write code to parse the string dates. In the RDFa example, one could link several sites on the shared purl.org URL of, e.g., Friday, to answer questions like "which restaurants are open on Monday". That's the whole Linked Data idea.
On the other hand, the RDFa way is more painful when the opening hours differ between the days of the week.
The datetime attribute is designed to be easy to parse. They stick to a consistent format on schema.org. They reference ISO 8601 which is quite a bit more complicated, but hopefully they'll add something saying they only support a tiny subset of what ISO 8601 allows to make it easier to write tools.
https://gist.github.com/1005688
I like this quite a bit better.