Only e-ink displays are excellent in the sun, but they have too many disadvantages currently to be used on general purpose tablets. Although I did read somewhere about a colour e-ink tablet that was being worked on.
OH has, for the last week, been using her Transformer to read books in full Florida sun with no problem, although it's still not as good as a Kindle in the same circumstances though.
I've used ipad2, transformer and vega in sunlight and out of the three the vega is pretty useless in direct sunlight, the ipad and transformer are OK. As psionic mention, the only screen that works well are e-ink screens like kindle.