Question: I'd like to mark a tableview row in some way that shows it has been clicked by the user. I have a large number of rows and want users to know if they have already visited a particular row. This same technique will be useful for identifying row new entries as well. 

I have two questions: Would such a subtle technique violate the HIGS and what would be a nice way to do it without muddying up the UI?

Answer: 
While my guess it that no, it doesn't violate the HIGS (they use both of my suggestions below in different apps, themselves), you never can tell with Apple!

Background color can be good for such things, depending upon the nature of the table. 

Another choice is to have a bullet which can be in three states:

* absent
* unfilled circle
* filled circle

Example here is the Podcasts in your iPod: they start out full, then go to partially full once the podcast is partially played, then go away once you've listened to it to the end.

[alt text http://www.me.com/ro/bencox76/Galleries/100014/Security%20Now%20Screen/web.jpg?ver=12579803280001](http://www.me.com/ro/bencox76/Galleries/100014/Security%20Now%20Screen/web.jpg?ver=12579803280001)

However, in this case you have two different meanings you are trying to convey. I might combine them - maybe full circle on unvisited rows, no circle on visited, and a special background color on new ones?

Good luck!
Maybe you could use a subtle background colouring of the rows that have been already visited.