I think it has been told before, but…
Please help. Could someone describe how to use the new mirror modifier, then join the two halves and remove the seam?
Yeah, can’t wait for 2.4 to be released 
Thanks in advance.
hi stereo 3d
I will try to explain how to use it.
Lets say we want to mirror default cube.
Delete the face pointing left.
Move pivot to the left so much that it is
not inside the cube.
add modifier -> mirror
alter merge limit
so that the cubes are joided.
Not too much merge limit
or it will mess up a model
when modeling.
Adding to j’s response:
You want to have a ‘centerline’ of vertices that will be the seam that will get joined. You can scale the centerline on one axis (s, x, 0 for ex.), so it will be more accurate and easy.
Also, put the mirror modifier first! I found out that if I put for ex. a subsurf modifier before the mirror, then I won’t get a perfect join, because the subsurf will ‘bend’ the vertices away from the centerline. (yes, I know you can fix it, but it’s much easier to just put the mirror first).