For reference, I am not a lawyer, trademark or otherwise. What I am posting is mostly common sense stuff.
Trademarks are there to protect a company and allow them, and them alone, to conduct trade (providing goods or services) using a recognizable mark (usually a word, though I think images can be trademarked).
If you use their registered mark (ie the word "google"), you are ingfringing on their exclusive right to do business under that name. The reason this is considered infringement is because a) Potential clients or customers may view you as being the real google, or b) Your use of the name google implies that you are a representative of, connected to or otherwise associated with google. In plain english, using an established trademark "tricks" your users into thinking you are somehow officially related to the "real" trademark owner, when you are not. It causes confusion.
There are times when use of a registered trademark is perfectly legal, without consent from the trademark owner. These include Parody and education, and I *think* legitimate complaints against the trademark holder.
Parody is when you make fun of a trademark or copyright, or it's owners. This can be a touchy subject, and I'm not really familiar with all the ins and outs of what is considered parody, but from what I gather, if it can be viewed as damaging in ANY way to the trademark owner, it's a no-no.
Education. Fair use is granted when you want to teach courses on new or emerging technologies that might be trademarked. For instance, if you are a college, and you want to offer a course on google adsense, it's much easier to say "Google Adsense Course" than it is to say "Course on that advertising program run by that company that has a really great search engine". This is considered fair use.
Again, let me point out that I am not a lawyer, this is not to be considered as legal advice, and if you are in a situation where you may be infringing someone elses trademark, seek the services of a lawyer.
On to the fun stuff.
The internet. What a wonderful place. Not only has it revolutionized communication and information management, it has thrown a wrench into the great machinery of patent, trademark and copyright law.
The internet is global. Anyone from anywhere on earth can view a website from anywhere else on earth.
Before the internet, People had to be within your viewing or advertising radius to even know about you. This allowed small businesses in disparate cities to operate under a name that may or may not have already been trademarked. For instance, In the make believe town of Whatever, Oklahoma, there may have been a store called True Value. They were allowed to continue doing business under that name, not because the "real" True Value Corporation (who owned the trademark) decided to let them use the name, but because their target market was local, and the "real" True Value didn't have an outlet in that town, and thus didn't know of the infringement.
Now introduce the internet. The Whatever, Oklahoma company wants to go global, and sees the internet as the means to an end, so they create a website "truevalue.com" and post their inventory on it.
Now, the True Value Corporation comes along, decided this internet thing is neat, and wants to do business on it, so they try to buy the domain name, only to find it's already been taken. A Battle ensues. The winner? The Trademark holder.
Why? Because they're better at what they do, or provide better service? No, because they have reserved the RIGHT to use that name for doing business.
Now look at the domain atgoogle.com. The domain is being used based on the strength of the "google" trademark. Face it, if the word "google" wasn't well known, you'd have chosen a different domain name.
Now, couple with that the fact that you are NOT doing parody, and you are not providing legitimate education, and you have a solid case of trademark infringement.
Here's the clincher: Don't expect to hear anything from google right away. If I were google, I would wait until your site got popular, and got a decent amount of traffic, and THEN I would file a suit. Let you do the work of gathering the traffic and users. If your site doesn't become popular, it's of no interest to google at all.
This works doubly so for internet based companies. If you are infringing on the rights of an internet based company, and they can double their traffic by taking a domain from you, expect a lawsuit, or at least an ICANN query to strip you of your infringing domain.
In the long run, it's best to avoid domain names that include trademarked words.
I hope this wasn't too confusing, it's almost 2am here and I've been drinking
