public const int CHUNK_SIZE = 4096; 
public const string UPLOAD_URI = "http://localhost:55087/FileUpload.ashx?filename={0}&append={1}"; 
private Stream _data; 
private string _fileName; 
private long
_bytesTotal; 
private long _bytesUploaded;   
private void UploadFileChunk() 
{
    string uploadUri = ""; // Format the upload URI according to wether the it's the first chunk of the file
    if (_bytesUploaded == 0)
    {
        uploadUri = String.Format(UPLOAD_URI,_fileName,0); // Dont't append
    }
    else if (_bytesUploaded < _bytesTotal)
    {
        uploadUri = String.Format(UPLOAD_URI,1); // append
    }
    else
    {
        return;  // Upload finished
    }

    byte[] fileContent = new byte[CHUNK_SIZE];
    _data.Read(fileContent,CHUNK_SIZE);

    WebClient wc = new WebClient();
    wc.OpenWriteCompleted += new OpenWriteCompletedEventHandler(wc_OpenWriteCompleted);
    Uri u = new Uri(uploadUri);
    wc.OpenWriteAsync(u,null,fileContent);
    _bytesUploaded += fileContent.Length; 
}   

void wc_OpenWriteCompleted(object sender,OpenWriteCompletedEventArgs e) 
{
    if (e.Error == null)
    {   
        object[] objArr = e.UserState as object[];
        byte[] fileContent = objArr[0] as byte[];
        int bytesRead = Convert.ToInt32(objArr[1]);
        Stream outputStream = e.Result;
        outputStream.Write(fileContent,bytesRead);
        outputStream.Close();
        if (_bytesUploaded < _bytesTotal)
        {
            UploadFileChunk();
        }
        else
        {
            // Upload complete
        }
    } 
}

有关完整的可下载解决方案,请参阅我的博文：File Upload in Silverlight – a Simple Solution