I guess the lack of responses speaks volumes!
Yes, the US Marshals are users of the IWN system. For the last several years, the Marshals have been partnering with the FBI's communications network, and since the FBI has been a primary user of the iWN since it went on the air, the Marshals Service is on there too.
There are some conventional, simplex channels that are available for the Marshals to use when out of range of the IWN sites, but I couldn't tell you what they are using in the Seattle area for sure...
- Chris