Jax has an enthusiastic post entitled “Building brands on the web“. There’s some great points in there, worth a read (it’s pretty snappy – so won’t take long).
In relation to point 1: “Web users are an active audience” – I would heartily recommend checking out Clay Shirky’s talk at Web 2.0 – it’s a goldmine.
And extending point 5: “Easy and cost effective device to gain customer insights” – make sure you have permission! I know Jax knows this, but didn’t state it explicitly :)
In other words, just because you have an email address and an “opt-in” checkbox doesn’t mean that people won’t consider you spam.