Note: This post contains my opinion and logic. It in no way is a legal representation.
Registering a domain that contains the words 'GoDaddy' when GoDaddy is a registered trademark instantly means that you are using someone elses name without permission. Sad, but true. But then come to think of it, what potential use would you ever have to register a domain that contains someone elses trademark?
The key part here, in my opinion, are the words "without permission". You can potentially get away with using a domain name that has a reference to company, providing you ask first and state that the content on the site is in no way linked to the original named company. Say for example you have Cpanel hosting, and want to start a site called CpanelFAQS.com - I'm sure they'd be more than happy to let you own that without any problems, providing you clearly state that it has nothing to do with Cpanel at all.
Is Cpanel even a registered trademark?
Then I think we have another issue of how common the names involved in the domain are. Example: Go Daddy is a pretty unique combination of words, is it not? You're hardly likely to EVER use those combination of words, in that order, unless you're referencing the company itself. How often do you say "Go Daddy!" to your father? Exactly, not very often.
In those instances it's highly obvious you're ripping off their trademark in one way or another. Unless you have an amazing lawyer and can seriously prove otherwise.
Let's look at your example GoFatty.com. For the technical minded amongst us we may be reminded of GoDaddy and think of it as a spoof site. I think in this instance the actual content of the site is key here. If your site has a similar design layout, similar content, or perhaps just spoofs their entire site then I think it's obvious that someone may have a say in things. However, if the domain is about fat people losing weight, for example, then it's sheer coincidence that it sounds similar to GoDaddy and I very much doubt that any court of law would compare the two seriously.
At the end of the day it's down to common sense. Don't bother trying to register a domain that contains another big companies trademark unless you are confident that the trademark itself isn't specific or unique enough to warrant them singling your site out, and that your content is completely different to the named trademark. The recent example GoDaddyNS.com is blatantly in breach. DaddyNS.com however could have been a different matter. Think before you register.
Just my 2 cents.
