Anything you do to provide "encryption" that isn't using SSL/TLS is likely to be vulnerable. Now you have to ask yourself, is it worth burning dev hours you could be spending on features on a rubber-chicken security measure? Maybe it is.
.NET APIs like DPAPI and the Win32 crypt32 API make it easy to encrypt blobs of data with static keys. But how will your clients receive the keys? Any installed SOAP client will have to either have the key burned into its configuration, or receive it over the insecure Internet.
This is the problem SSL/TLS solves for you; the dance you do with TLS certificates is what solves the problem of communicating public keys over untrusted channels.