The N900 stylus will fit in the N810. The "head" looks a little mis-matched in the opening when it's all the way in. But it does fit all the way in, it doesn't stick out, and the inner friction clip does grip it so it's not likely to fall out.

On the other hand:
The N810 stylus will slide into the N900. But it's not a true fit. It doesn't go far enough in for the friction clip to keep it from dropping out. And the head sticks out from the body of the N900 because the head is a little bigger than the head on the N810 stylus.