Hehe, we've just had a big "fight" (discussion, but for an outsider our discussions often look like we're having a fight ;)) about it.
We decided not to do so: most people (or at least, the people we're marketing to) will probably fill in their real address anyway.. And if they don't, they're smart enough to use a "dump"-address.
Pros: You are able to send email updates to users.<p>Cons: Duplicate users.
Misdemeanor: Imagine if someone uses the whitehouse or fbi email addresses. Then he keeps on clicking on "Retrieve password" and you would start spamming the whitehouse and fbi.
A great solution is to give them an incentive to give you their real email address, like you're going to send them an important report or something else that is of high value so the visitors have no problem giving up their email address.