Well, the contact move is a middlegame invasion joseki, like the 3-3 invasion or the submarine invasions at 2-4 or 2-5. It has more in common with these moves than it does with the keima approach, which is more of an opening joseki.

The difference between the contact move and san-san is that it aims more as settling on the edge than in the corner. San-san allows the opponent to choose the direction of play and is best used in situations where that isn't such an advantage. When one side is clearly worth more than the other, the contact move may be better as it allows the invader to choose the direction (since he will usually end up settling on the side on which he played the contact move).

The submarine invasions are similar to the contact move in that they have a definite directionality to them... when to choose the submarine and when to choose the contact move is a question I've never had a good answer to, so hopefully someone stronger than me can enlighten us both.