Why my photos end up half dark? Sometimes my motorola milestone camera goes crazy and starts out to take photos half dark.

I noticed that it only happens when flash is on. Also, I've seem this happening when the battery is low (~20%), or the system is slow.

As an example, this is a photo I took minutes ago: http://twitpic.com/2q65bs
Take it back to wherever you bought it and demand a refund.  Regardless if this is a hardware or software problem, you shouldn't have a problem with the sales staff -- show them the picture you linked to.