I just bought three Velocity Dyad rims and am confused about the weights. Every website I've seen, including Velocity's own, lists these rims as 480 grams for the 700c version. However, the two (silver) 40-hole rims that I got for the tandem weigh 522 grams each, and the (black) 36-hole rim, for my cyclocross rear wheel, weighs 542 grams. I then weighed the (black) 40-hole rim that came off of our tandem (a 2008 Co-Motion Speedster), and it weighs 452 grams! If you're wondering why we need a new rim for our 18-month old tandem, then see this thread about our encounter with a narrow drainage channel. The second tandem rim is a backup in case anything like that happens again.
I know actual weights don't always quite match the list weights, but I've never seen deviations of between -5% to +10% (or more). AFAIK, the main competitors for a rim in this sort of size and strength are the DT Swiss TK 540 (previously known as TK 7.1), and the Mavic A319 or A719 (which are similar to the old T519 I believe). The DT Swiss is listed as 540 grams and the Mavic A719 is 567 grams. So, the Velocity used to be significantly lighter, but maybe that is no longer true (but maybe the list weights of the other brands are equally inaccurate).
The other thing to note about the Dyad rims is that they are not even distributed in Europe. I spoke with Brick Lane Bikes in London, UK, who are the European importer for Velocity rims. However, they only import certain models, basically whatever they can sell to the fixie crowd in London. They were not at all interested in obtaining some Dyads for me. I therefore ordered from an online store in the US. It took 3-4 weeks, but I now have them. Shipping (to Switzerland) was US$50 plus import fees, which were the equivalent of US$15 (plus local sales tax). Fortunately, I wanted to add a fork to the order, so that made the flat fees a bit more worthwhile. I was hoping that they would decide to start importing these rims to Europe soon, but now it looks like it is not so important - I'll just go with the local brands of Mavic or DT Swiss next time.