Although I agree Richard Dawkins can be a little strident and sometimes "opens mouth before engaging brain" (as my Dad used to say), the definition of "fundamentalist" is: "a form of religion, especially Islam or Protestant Christianity, that upholds belief in the strict literal interpretation of scripture". Since Richard Dawkins is not religious and therefore doesn't base his views on any scripture, I don't see how he or any other atheist can be described as a fundamentalist.
I actually have no problem with Jehovah's Witnesses - I usually find them very pleasant and I understand that they feel they are doing a good thing in trying to "save" me. But I'm not sure how much they'd appreciate me knocking at their door to talk about atheism - similarly the people who stand in our town square at the weekend and shout about the devil, hell and such things.
Having said that, I do think that the people who write such letters to Richard Dawkins, or anybody that they feel is being irreligious, have either a mild learning disability or are mentally unstable and I'm not sure it's fair to imply that they represent the views of mainstream Christianity. Apart from those who are obviously disturbed, I think that anybody who never questions their own beliefs (not necessarily religious beliefs) can, I think, be a dangerous person.