IDM is sort of a residual category, isn't it? the electronic music that doesn't fit into the other easy-to-define genres? or is it a specific sound with specific rules or markers?
seems to vacillate between those poles, as i see it. but i think jay's point is valid to an extent...for many practitioners, the point of IDM is that it is self-consciously "brain music" and not necessarily self-consciously "dance music." by "brain music," i guess i mean music that is designed to be listened to and appreciated on its own merits, not meant specifically for a dance floor and not necessarily meant to be DJed. "dance music," then, would be music composed specifically for people to dance to and to be DJed. techno, as i see it, is almost always self-consciously "dance music" and only sometimes self-consciously "brain-music."
[note: you can argue techno is always "brain music." i'm just saying that a lot of techno producers think of their music as "for the dancefloor" rather than "for the living room or ipod]