Discrepancy in well-formedness of XML between Flex 3.4 and Flex 4.0 (Server error: Data at the root level is invalid)

Steps to reproduce:
1. Compile project using the Flex 4.0 SDK
2. Pass XML using either the toString() or toXMLString() methods into an ASP.NET Web service that is expecting a String data type in the form of XML as in:

<request><arg>value</arg><otherarg>value</otherarg></request>

Actual Results:
When I compile the AIR project using the 4.0 SDK, the .NET Web service returns the following error:
System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.

Expected Results:
It should not cause the Web service to throw an error

Workaround (if any):
Compile the very same code using SDK 3.4 and the ASP.NET Web service accepts the string as a well-formed XML string and returns the data.

I realize that this may be difficult to reproduce but it just seems strange that I would lose interoperability with the Web service just from compiling with Flex 4.0 and am hoping someone might be able to shed some light on a workaround that will allow me to use SDK 4.0.