Okay so I finally have it figured out thanks to the nice people at microsoft.public.win32.programmer.directx.managed newsgroup.

I'll just post the code here too in case someone else might someday run into a similar problem, and to be honest, there are no good examples of doing this unless you're using a Mesh object.

//Checks if point P is inside the plane

internal bool PointIntersects(Point p, Device d, out IntersectInformation intersectInfo)

{

//Create near and far vectors from the Point location

Vector3 near = new Vector3(p.X, p.Y, 0);

Vector3 far = new Vector3(p.X, p.Y, 1);

//Unproject the vectors into world coordinates

near.Unproject(d.Viewport,

d.Transform.Projection,

d.Transform.View, Matrix.Translation(location));

far.Unproject(d.Viewport,

d.Transform.Projection,

d.Transform.View, Matrix.Translation(location));

//Calculate the direction of the ray from the vectors

Vector3 dir = Vector3.Subtract(far, near);

//Set up the locations of the vertices

//in the first triangle in the quad

Vector3 loc1 = location; //Bottom left

Vector3 loc2 = new Vector3(location.X,

location.Y + size.Height, location.Z); //top left

Vector3 loc3 = new Vector3(location.X+size.Width,

location.Y, location.Z); //bottom right

//check for intersection

bool intersection = Geometry.IntersectTri(loc1, loc2, loc3,

near, dir, out intersectInfo);

//If we didn't find an intersection in the first triangle,

//check the second

if (!intersection)

{

//Set loc1 to the coordinate of the top right vertex

loc1 = new Vector3(location.X + size.Width,

location.Y + size.Height, location.Z);

//Check the other triangle

intersection = Geometry.IntersectTri(loc2, loc1, loc3,

near, dir, out intersectInfo);

}

return intersection;

}

I used that inside my Plane class which is used for rendering 2D-like quads.