Yeah, I just heard about this on the news yesterday. I had considered posting a thread about it, actually, but it's nice to see that someone else stepped up and did the honor.
I think she is very gifted and I like quite a few of her songs, but I don't expect her hiatus to last
too long. It's hard for artists, sports stars, etc. to stay away from what they know and love for an extended period of time. I give it a year or two tops before she comes back with more.
A singer named Ayaka recently stopped singing as well (just three years after debuting) after being diagnosed with Graves' Disease. YUI (one of my favorite singers) also went on hiatus for a while, but it didn't take her very long to come back (thank God!

).
Utada Hikaru has been trying to break into the U.S. market for years now; I'm sure she's disappointed that her attempts haven't been very well received.
No, I'm not saying that's the reason for her hiatus.