Can Persistence Ignorance Scale?

I've been having a brief look at NHibernate and Linq2Sql. I'm also intending to have a peek at Entity Framework.

The question that gets raised, when I talk of these ORM's is "They can't scale", so can they? From Google I get the impression they're able to scale well, but ultimately I suppose there must be a price to pay, Is it worth paying for a richer simpler business layer.