When I first joined SL - some years back - I was in an RL relationship and did not look for one in SL. I was happy just wandering around like a noob, RPing, dancing meeting new friends etc. When my RL relationship ended I found myself spending more time in world to distract myself and spent more and more time with a friend which turned into an SL relationship, which in turn ended up being an RL relationship. We ended up partnering in SL and still continued to log in together and talk to friends, RP and what not.
I guess I answered all questions in one. Yes, I would meet my SL SO if I felt the relationship was strong enough. I don't think the online relationship changes much. I guess the only thing that changed for me was the amount of time I spent logged in.
I am currently in an SL relationship and although I have not met him in RL, I will some day soon. I don't really class our relationship as an SL one because we technically got together outside of SL and had not even met in world at the time. The joys of friends introducing friends through plurk and skype. But now that we have met in world we don't tend to leave each others sides for all that long. Unless we go shopping and I spot shoes somewhere then I will abandon him at the drop of a hat to go in search of a bargain.
The only thing I can see changing for us online when we finally do meet, is the amount of time we will spend not logged in. The only reason we both log in at the moment is to see the other one. But most of the time we are in skype. I guess for long distance relationships like ours, and others that I know, SL sort of serves as a way to 'be with each' when you can't. It may jut be pixels sitting on a cuddle poseball...but sometimes that's all it takes to turn a frown upside down.