Maybe my case was just an exception, but I had been to the UK 3 times before I went over in 2009 to stay for a while. I don't remember exactly what I told the IO, but I told him I didn't know for sure when I was going back but that I planned on being there for a few months to be with my boyfriend so we could see if our relationship worked on a day to day basis. I did NOT have a return itinerary, but I was traveling there with my mom and my sister and my sister worked for the airlines (I don't know if this matters). He asked what I did for a living and how I would be supporting myself and I told him I had savings and my boyfriend would support me as well. He did give me the evil eye for a few minutes and took his time deciding, but they stamped my passport for 6 months and I stayed for 5 and a half.
I'm not sure if this will weigh negatively on my visa application now, but I have been back to the UK once since then for a week and underwent 15 minutes of intense questioning and had to show my return ticket back. But they stamped my passport for 6 months again, so I'm assuming all is well.