I'm going to use a non-real example to protect all parties here. Let's say I really like Google. I like it so much, I make a community website for google and register the domain googlecommunity.com Does google have any legal right to force me to choose a new URL or have a say in what kind of ads are displayed on my sight? For example, let's say I use yahoo ads and it displays ads about a new search engine instead of google.. Does google have a right to force me to stop this? Another example would be Pepsi.. I looove pepsi and make pepsilovers.com. Can pepsi force me to change this domain? What legal rights do I have? I'd also like to be pointed in the right direction to find great trademark/domain name/internet legal counsel.
Yes. Yes. Yes. Legal rights are challenged every other day. There are lots of lawyers who are versed in domain-trademark disputes like John Berryhill, Enrico Schaefer, Brett Lewis, Howard Neu, Martin Garthwaite, and a whole lot more. (just search for any of them via Google or so...) Shorthand is expect trouble if you register a domain name bearing a trademark that's heavily protected by their holders with deep pockets, especially if it will likely cause confusion. Likelihood of confusion is where trademark infringement boils down to.
The matter here is whether or not you are infringing on their trademark. (You infringe on someone's trademark if you use their trademark in the same business and area they do business in.) If you inhibit their ways of making money in any way using their trademark, they could probably get a justifiable lawsuit open. If you are just talking about how awesome Google (or whatever) is, then I would think it is okay. Best of course is not the use their trademark at all. Hope that helps.
Your best bet would be to stay away from using a trademark in a domain name when the sites content is related to the trademarks holders usage (domainsucks.com would be an exception) unless you obtain permission. As stated, yes, they can sue you for infringement or take the domain in a WIPO action. The type of ads you display don't matter - although in google's case, you would also be violating their adsense TOS - which is really a different matter. You will have no problem finding an intellectual property rights attorney to represent you. Be prepared to put up a $5,000 to $10,000 retainer and TM suits can wind up costing well into six figures. If you lose, you can also be required to pay damages, treble the other sides legal fees, and be forced to turn over the domain. If you win, you will likely not be award attorney fees. That is one of the advantages of having a registered trademark.
Personally, I would stay away from having a domain where there is any concern about a trademark issue, at least when you are talking a well-known company as in your examples of Google and Pepsi. You are just setting yourself up for all sort of potential headaches and serious legal expenses. If you do decide to go that route, definitely consult with an attorney who is very experienced in dealing with the laws related to trademarks. As someone mentioned earlier, big companies have very deep pockets and a legal department, so it just doesn't sound like you would have much chance of winning if there was a legal dispute, and if by chance you could or did win, the cost would probably not be worth it, IMO.
I would stay away from Trademarked domains, even in the cases above. It is Infringement, and even if you are helping the business they might still take you down. If you are concentrating on Search Engines, I would suggest something more like searchenginecommunity, dot com or whatever domain you choose. Then you can mainly focus on Google, or Yahoo. Good Luck!