For virtual modeling/whiteboarding, there are ample tools available to allow participants to collaborate as if they were physically together in front of a board. The key is using a digital pen versus using a clumsy mouse to act as a whiteboard pen. If your physically collocated teams use a physical task board then create one that includes your distributed members as well. Some teams have used Visio or Word (with shapes). I helped one team build their own that was specific to their SD methodology and supplied metrics important to them yet it functioned just as their physical task board did in terms of moving items (index cards and other artifacts) around into various columns representing status and other categories of information. Use webcams at least occasionally. This can add an often needed dynamic, especially in the early stages of development.