Quoting Ace_of_Sevens:

Quote:
HDTV uses a completely different captioning system which is not available on Blu-Ray.  As used in DVD Profiler, it specifically means line 21 captions.  Anything else goes under subtitles.

Wikipedia wasn't my initial source, just a handy link for the info.

The wikipedia entry is wrong/incomplete.
A variant of Line21 is allowed in digital encodes as others have already said.

And CC is supported in high definition broadcasts (e.g. some HDNet broadcasts and lots of OTA TV shows and late night) and BD/HD DVD encodes as well. E.g. some FOX BDs have embedded CC as well.