SWFObject is a free set of code that can help. It consists of an Adobe AIR application (or if you don't want to use AIR, they also offer a web page), which lets you fill in info about your Flash movie and what alternate content you want to display when Flash is not supported by the user's device and/or browser. SWFObject then gives you the appropriate HTML markup to include. You also need to upload the SWFObject javascript file (swfobject.js) to your site and link to it by adding the supplied script statements to include in the head section of your web page.

Here's 3 links to SWFObject:
http://code.google.com/p/swfobject/wiki/documentation
http://www.adobe.com/devnet/flashplayer/articles/swfobject.html
http://blog.deconcept.com/swfobject/ (relates to an older version of SWFObject, but still has useful info)

And here's a link to a free DotNetNuke module called DigFlash that uses the SWFObject code, and is designed to help DotNetNuke developers easily embed Flash movies on DotNetNuke sites:
http://www.dignuke.com/Modules/digFlash.aspx

However, I'm not sure if they've updated it to use the most version of SWFObject, because as of 5/1/2011, they were still linking to the page for the old version of SWFObject on blog.deconcept.com.

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