I don't think DX10 is dependent on Longhorn at all. It's not like Win XP won't support DX10, and Longhorn's graphics engine is built around DX9.
In other words, Longhorn will likely ship with DX10, but I don't see DX10 being held up because of Longhorn.
That being said, DX10 could be held up for other reasons: IHV support. If PS 3.0/3.0 has as long a "life span" and PS/VS 2.0, that would put DX10 hardware debuting in late 2005.