Hello Joris Poelmans,
Let me know if I understand the problem correctly. You can see the Flash, when you are in Edit mode and you are toggling between Design and Html mode of the RadEditor and you can see the HTML code of the flash object, when you are in Html mode of the RadEditor. The problem occurs after you save the page. The flash object is stripped.
If this is the case , this seems to be yet another problem, caused by the MOSS content filter. When you submit the page, MOSS will validate it and automatically make changes if the content is not valid. You can verify this by trying to enter the same content with the original SharePoint rich text editor.
We have not been able to find a workaround for this behavior yet.
the Telerik team