Unfortunately, no. SlickUpload doesn't use any ActiveX/Java/Flash plugins, so it is limited by the functionality of the browser. We endeavor to squeeze out as much functionality as possible, but some things are confined by the inherent limitations of the browser. IE up to version 9 only supports single file selection, no multiple selection or folder selection. IE 10 is looking like it will support multiple file selection at the least, and potentially folder selection as well. As soon as it is released, we'll add support.
Right now, multiple file selection is supported in Firefox 3.6+, Chrome 8+, and Safari 4+. Folder selection is only supported by Chrome 11+.