There are some known issues with swfobject and ASP.NET, infact it’s not just with swfobject but also with the Flash object in general, one issue of using ExternaInterafce from an ASP.NET Form can be solved with these technics

I had a strange issue with swfobject lately and obviously I’ve blamed ASP.NET for inserting unwanted code into my pages and causing problems. Generally it’s reasonable to blame it as it does make a mess sometimes, but, this time it was my fault for not noticing other Javascript code is conflicting with swfobject.

The issue I had, was with the swfobject’s addVariable and addParam functions. The Flash SWF HTML seemed to be written to the page’s flashContent div but all of the variables and parameters I’ve added were ignored. After examining the swfobject getSWFHTML function, this function gives you the HTML code that is gonna embed the Flash inside the page, when I saw how strange the HTML is, I realized what happened:
Without naming names some Javascript developers, extensions and frameworks like to write to the prototype of generic Javascript objects (This is also how Object Oriented Actionscript 1 was done in the past). And with doing so, extending these built-in objects (object, array, string, etc’) with various functionalities. A good example is the javascript JSON implementation which extends the Javascript object with object.toJSONString(). Swfobject stores the variables and parameters inside a regular Javascript object and when it prepares the Flash HTML it uses a for..in loop to go through all the elements and add them to the markup
<param value=”flashMovie.swf” name=”movie” />
<param value=”transparent” name=”wmode” /> etc’

in case you’re using the json.js, your HTML will have also
<param name=”toJSONString” value=”function (w) {….and whole lotta mess” />.

This might cause the embedding of the Flash movie to fail or function improperly.

The solution for this is to add a check to all of the for..in loops inside swfobject with the hasOwnProperty, for example:

[JS]for(key in variables){
if( variables.hasOwnProperty( key ) )
{
variablePairs[variablePairs.length] = key +”=”+ variables[key];
}
}[/JS]

The hasOwnProperty function returns true only if the property is not built-in and not in the prototype chain. Therefor the toJSONString in our example will return false and wont be considered as a flash variable or parameter.

When encountering issues with the swfobject a good place to check is the swfobject.getSWFHTML() function.
[JS]var o=new SWFObject(“flashFile.swf”,”falshMovie”,200,300,”9″,”#FFFFFF”);
o.addVariable(“firstName”,”Jon”);
o.addVariable(“lastName”,”Smith”);
o.addParam(“wmode”,”transparent”);

//exmine the html before it’s being writen to the div
alert(o.getSWFHTML());

o.write(“flashContent”); [/JS]

More related info about hasOwnProperty.