Lustrado Crocheted Pinwheel was crocheted with fingering weight yarn and 3.0 mm crochet hook. We think the curly problem might be caused by different weight of yarn and the hook.
My co-worker, Flora crocheted a swatch with elann.com Sonata which is DK weight yarn. It curls a little bit, but it is flat after blocking. In her experience, if it starts to curl, you can add extra stitches around that row, and even more in the next row. In this pattern, she suggested that when you do v stitch, you could do 2 chain stitches (dc1, ch 2, dc1) instead of 1 chain stitch as the pattern calls.
If your project tends to get curly a lot, can you please send us a photo so that we can look into it, and might be able to give you other suggestions?