Is that a DeepV rim by any chance? And since you mentioned Sheldon Brown, can I assume you're following his wheelbuilding tutorial? If so, and you're following the instructions to letter (key spoke immediately to the right of the valve hole), you will end up with the nipples pointing in the wrong direction on a DeepV. I had the
same problem recently. And the way your spokes curve into the nipples looks suspiciously familiar.
If you're going by Sheldon's tutorial, you may need to start with the key spoke on the second hole away from the valve hole. Sheldon's instructions cover this situation quite well, he just doesn't go into a lot of detail on how you figure out which side of the hub a particular nipple hole is meant for. I believe that his guidelines for determining this apply mainly to box section rims. With deep section rims it's trickier.
On many deep section rims the nipple holes are drilled at an angle, pointing alternately to one hub flange and then the other. When a rim is drilled this way, the holes on the rim tape side end up staggered to the opposite side of the rim from the hub flange they're intended for. In other words, a hole on the left is actually pointing to the right-side flange, at least on the DeepV. Again, I believe Sheldon's tutorial implicitly assumes a box section rim, because they generally have the spoke holes on the same side as the intended flange. It's easy to make this mistake if you aren't aware of the issue.
Roger Musson's wheelbuilding book (highly recommended) covers this.
One thing you can do to be sure is to drop a nipple into a hole and loosely attach a spoke. Wiggle it around a little bit and you should be able to feel the natural line of the drilling.