This problem should be fixed in the latest release (4.41) of the MOSS RadEditor control. If you upgrade to the latest version, the MOSS asset picker dialogs will load normally.
If you do not wish to upgrade at this time then you should be able to workaround the problem by copying two files on the MOSS server. Navigate to the \Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\
folder and create a new folder with your server's locale ID (for example, the English - United Kingdom lcid is 2057). Copy the AssetPickers.js and HtmlEditor.js files from the 1033 folder to the new folder (2057).
I apologize for the inconvenience this has caused you.
All the best,
the Telerik team