Warning: Slightly long opinion piece here...
In tech blogs, comment sections on most sites, and forums, ****** seems to be used/seen as derogatory for the most part. When I see someone write "He's an apple ******" for example, I might immediately think of someone who blindly purchases all Apple products without a second thought, making excuses for nearly every shortcoming they may have or mistakes they may make, may be seen publicly "defending" Apple products and the brand as aggressively, word-twistingly, or blatantly as they can (sometimes the truth, sometimes an exaggeration, or sometimes only believing/hearing and regurgitating what they want to see), over exaggerating the usefulness of any minor new feature/product/update and... basically... defending them against all odds.
It may seem a little extreme, but I feel that is what many think of when they think of a "true ******". It extends beyond smartphones, OEM's, and Apple/Microsoft/Google/RIM... a "COD/Call of Duty" ****** would be someone who blindly purchases every new Call of Duty game... and defend it against other competing shooters such as Battlefield.
Either way, I think there are varying "levels" of fanboys. Aside from the "true ****** / extreme ******" mentioned above, there are the people who are simply loyal to a company but know its faults and shortcomings and accept it hoping for improvement - maybe even getting actively involved in a petition or something - but they will still usually buy many of their products. Then there are those who may be loyal to a company but accept that they can put out some crappy products that aren't for them so they get what does work for them - even a competitor's product - yet still publicly defend the company for which they are loyal to and hope for the best. Oh, and then you have the fanboys that aggressively defend their company of choice until they find something that really pisses them off about a new product. After some whining, they pledge their allegiance to another company who "never would have made a product like that" and they may or may not stick with that one either
Finally, you have the "casual fanboys" who are usually just defending their purchase because - c'mon - you don't want to look like the doof who spent money on a crappy product. As an example, my mom turned into a "casual ******" of the Android platform because all of her friends and everyone she talked to would not stop raving about Android (same with my dad, actually) so she bought one. When I got my first WP last November my mom asked how I was enjoying it, but every feature I figured would be awesome to demo she would retort with, "Oh yea, my Android does that" and it got annoying... can't I be excited about my new phone? I would show off a feature that Android can't do natively without an app - or that I think WP does better - and I will get the "Well, I'm sure Android can do that, I know there are a lot of things I don't know about the phone" speech. Of course, after her phone started getting sluggish and slowing down, the attitude changed a bit. I reminded her that, with Android, she'll have to get the top of the line phone to ensure a smooth, non-laggy experience... a bit fanboyish of me, but hey, I experienced that on my old Android phone
I also reminded her that there are other smartphone platforms out there, and each has its faults.
All my opinion of course, based on my observations in the tech community and the general public's growing allegiance/loyalty to companies and their products. . I don't think the word "******" will always carry a negative connotation. As an example, I casually refer to myself as a geek but if I say that in front of my father he gets upset and tells me not to insult myself like that. Then again, geek can still be used negatively depending on how someone says it or in what context. So can any word.