swfupload.js に

// WARNING: this function does not work in Flash Player 10
// Public: selectFile causes a File Selection Dialog window to appear. This
// dialog only allows 1 file to be selected.
SWFUpload.prototype.selectFile = function () {
this.callFlash("SelectFile");
};

// WARNING: this function does not work in Flash Player 10
// Public: selectFiles causes a File Selection Dialog window to appear/ This
// dialog allows the user to select any number of files
// Flash Bug Warning: Flash limits the number of selectable files based on the combined length of the file names.
// If the selection name length is too long the dialog will fail in an unpredictable manner. There is no work-around
// for this bug.
SWFUpload.prototype.selectFiles = function () {
this.callFlash("SelectFiles");
};

と書かれているが、Flash Player 10をダウンロードしてテストしてみたが無事動いた。

http://demo.swfupload.org/v220beta3/simpledemo/index.php
でも動作確認。

flash バージョン 確認方法は、
http://www.atmarkit.co.jp/fwin2k/win2ktips/955getflshvr/getflshvr.html