Digital Photography Review has just published an excellent report titled, Compact Camera High ISO modes: Separating the facts from the hype by Simon Joinson. Simon states early in the report:
"Perhaps unsurprisingly there's a lot more to high ISO performance than mere numbers, and - particularly with the latest 6-10MP compacts - we believe consumers are being misled by the manufacturers when it comes to the actual performance of most cameras at higher ISO settings and in low light. So we decided to put together a short article on the truth behind those headline high ISO modes."
Reading this article is time well spent. You'll learn about the relationship of noise to sensor size, high ISO modes compared, and more. It's a very comprehensive piece.