Actually, the problem here is that you're using W3C valid HTML, which IE has a tendency not to get along with. To get IE to embed flash content you need to use embed, which does exactly the same thing as object but is not part of the W3C standard.
The syntax for the embed tag is as follows:
<embed src="somefilename.swf" width="550" height="400"></embed>
This is often just placed inside of the object tag, so that both options are on the page (don't worry, it won't be rendered twice in browsers that understand the object tag you already have).
A common way to get around using non-standard HTML is to do the embed with Javascript, which writes non-standard HTML code to the browser but not until after the page is rendered. This allows the page to pass the W3C HTML validator, and still work cross-platform. The best javascript library for doing this is generally SWFObject, which lets you just include the flash content once and will write html out for whatever browser the user is viewing your content in.