At this point of development of Silverlight, everything has to be done in memory, on both sides.
We are working at implementing a "SendChunked" feature for our component, just as the .NET framework provides for the HttpWebRequest class, but we have no time frame as to when this will be implemented. I must add that on the server side, even in .NET, the file is always all loaded in memory before you have access to it.
In principle, 100 MB should still work, if your server is configured to accept such large file, and that you have sufficient memory on each side.
André
Software Developer and Tech Support
Xceed Software Inc.