I have example how to work with sharepoint data from silverlight client:

```
 var tipSenderList = web.Lists.GetByTitle("List title");
                    tipItem = tipSenderList.AddItem(new ListItemCreationInformation());
         contactItem["FirstName"] = model.Name;

```

All data stored to sharepoint portal without problems... 
But how can i store multilookup field?
I tried:

```
 contactItem["Contacts"] = "1,2,3";

```

But was stored only first.... What can i do to save multilookup field?