Hi Stuart Hemming,
RadUpload is based on the <input type="file" /> HTML element and inherits all of its limitations. However this is the most cross browser efficient approach. Any other solutions rely on plugins - Flash or Silverlight. For example Google's upload relies on Flash to upload files. If relying on a 3rd party plugin is not a problem for you (keep in mind that your customers need to have that plugin installed) you can check RadUpload for Silverlight. We have assembled an integration demo
showing how to use it in ASP.NET applications.
the Telerik team