We are looking at an email campaign using an email list made up us addresses provided by our current customers. They have opted in.

Someone has put the idea in the head of our owner that the majority of the emails we send will not be delivered due to "SPAM blockers" put in place by the various ISP's.

1-Is this a valid statement?
2- If so, what are my options to maximize our campaign?

I plan on using a service like "Constant Contact"

FYI I'm no techie, so I'll take all the help I can get.

It is, but the spam blockers are by your customers' email client not by their ISPS. As your customers have given the email addresses knowing they would be receiving promotional emails from ur company, it would be ok. Never send promotional emails to random people's email addresses to prmote your business, this kind of marketing is not an accepted form..it would provide publicity but it would be adverse publicity which could turn out to be disastrous.

But since they have been willingly provided by your customers, one tip i now would be to use professional email subjects, this is one way spam blockers check for spam mails...

Care for SPF record (http://en.wikipedia.org/wiki/Sender_Policy_Framework) . Check whether your Hosting Provider has this.