here's an interesting paper on sharding with Oracle vs. Mysql
After seeing some real-world, big business problems solved with weakened consistency guarentees, I'm skeptical that there are as many problems that "need" ACID as most people think. Rather, I think that (a) most engineers have not yet been exposed to the reasonable alternatives to ACID, and so have not actually thought about whether they "need" it, and (b) most businesses do not yet have the knowledge they need to weigh ACID's benefits against its availability costs.
For balance it would be helpful to have an analysis from a more independent source than Oracle.
Sure sharding is scaling, although there are several types of sharding, with varying degrees of scalability. Usually people use a combination of both database (which what I was talking about) and table sharding (w/ replication,clustering,etc.) to achieve scalability. I've encountered several highly-scalable db environments like these.
It's not application-transparent
There are several databases that have features that can make sharding transparent to your application.
and it's an operational nightmare
it depends on the RDBMS that you're using.