I have the HA-11 Hood for my DP1. It works well but causes a problem. The square hood doesn't have a cover or accept any conventional type cap as near as I can determine. The problem then becomes deciding if it's better to remove the hood and carry it separately or to leave the lens uncovered. If the lens is uncovered it seems to actively attract dust which is hard to remove unless of course I remove the hood.
Fortunately the round section of the HA-11 is threaded and another camera's hood will screw into it, in my case the metal Contax GG-2 hood. Further, the GG-2 size is 55mm meaning the cap from my Tokina 100mm macro fits perfectly, more easily than the Contax cap. Now if only the GG-2 was threaded so I could use the damn fine polarizer I bought for the 100mm Tokina.
One side note, with this installed I am able to turn the DP1 on without worrying about first removing the Sigma lens cap. A small thing to be sure but to me a comfortable one.
With the multi color hood, the Tokina cap and the invisi shield plastic installed and peeling it sure is a raggedy looking thing. Fits me somehow.