This is a question I myself and many others ponder over, constantly.  Not that my techniques are unethical, but the fact that my focus is to beat the system and arrive at the top of search engines.  This in itself is a personal gain, regardless if it is your site or a client site which I'm promoting.  This is solely a purpose of personal gain and accomplishment.  Of course, I always have the site visitors in mind and I think of what will benefit them, as to how easy it is to find the site?  If they click on the link will they find what they are looking for?  Is the advertising practice intrusive or invasive? etc.  These are questions I ask myself, as well I choose a primary advertising platform which will best benefit the client.  Search engine optimization and ranking is usually primary or at the top of the list, and getting a site to the top of the SE's is not about benefit to the client or visitor (outside the realms of the mentioned), but it is a game between me and SE's which is a personal gain.  When you get a site to list at the top of a Search Engine it is like a breathe of fresh air and new ideas, you become overwhelmed with your talent and ability, every action becomes more precise and aggressive.  Though, many times landing at the top has nothing to do with skill and talent, it has much more to do with "pure luck".  Of course, you need some level of skill and knowledge.  And of course, there are niche markets out there that are in demand and not highly populated so it will be much easier to gain notariety within those markets, but the markets which are overly populated and harder to gain notariety are the sites which give you a sense of accomplishment when you break the barrier and is at the top of the search.

Now, back to ethics.  When reading the Google Webmaster Guidelines I sometimes feel that what I have practiced is unethical.  I do use link exchanges to promote sites and get a better ranking in SE's.  Though, I'm unclear about the Guidelines because it states:   Don't participate in link schemes designed to increase your site's ranking or PageRank.   This is something "I do not do" (I think), because "I DO NOT" participate in link schemes or any link farms which plaster links all across the internet on unrelated topics.  I exchange links with sites that are related in topic or I submit links to directories under a related category. 

Second, I had no idea (when I first read the Guidelines) that using computer programs to submit to Search Engines was not recommended by Google:    Don't use unauthorized computer programs to submit pages, check rankings, etc. Such programs consume computing resources and violate our terms of service. Google does not recommend the use of products such as WebPosition Gold™ that send automatic or programmatic queries to Google.

"Ouch, that hurts!", I wonder how well "WebPosition Gold" is doing after this announcement.  I hadn't used automatic programs as of yet...I have used online programs like 'addme' and others, but I am not clear if this is not recommended.  What is Unauthorized?  In Google terms, I'm assuming if they haven't paid Google for their approval!  Hmmm!  I had found a product which I believe is very good, but after re-reading Google Webmaster Guidelines I decided to hold off and do a little more research.  I do not want to practice anything unethical or UNAPPROVED.

Anyway, Silverdisc has a great article on SEO Ethics, titled Ethical Search Engines Optimization Explained".  It's worth reading if you have the time and interest.