I always feel like actual things have happened between them before. I can't remember if Casey met Lisa at 18 or 19 ('cause if it's 19 Casey knew Dana first), but I always get the feeling that Dana and Casey may have been close to getting together at that point, and then Casey ended up getting married to Lisa instead. It wouldn't have seemed strange to me at all if it had come to light at some point during the show that they had slept together in college or something.

I have a hard time respecting Dana's relationship savvy at any time except when she and Casey are talking/yelling in Intellectual Property. There she seems to realize that this is just a thing he does and that her thing with Gordon is just a fun thing. After that, though, she spends so much time trying to keep Gordon, who isn't good for her. It's actually funny; some of the time during season 1 I actually get tired of their attraction being a bit forced. During season 2, when they're flirting and whatnot during the first part of the season, I do believe that they both like each other a lot. However, in season 1 I don't get the feeling very much at all.

Also, from what happens at the end of season 2 ("in the last days of a war" etc.), I get the strong feeling that Sorkin was going to have them sleep together. I think that would have been interesting, although there's also the possibility it could have been done very badly.

I'm not sure there's a coherent message in this post, but I just had a bunch of different thoughts that came to me as I was typing... ;)