In the Summer '13 Release Notes on page 238 it states that web service endpoints in the form of
instance-api.salesforce.com must be updated to the new format:
API Endpoint Changes
Prior to Summer ’13, API endpoints for web services used a hostname of the form instance-api.salesforce.com. API endpoints now use a hostname of the form instance.salesforce.com. An example server endpoint URL from a SOAP API connection might look like: https://na1.salesforce.com/services/Soap/c/27.0/00DU0000000Q4f0.
If you have any code that uses a hard-coded API endpoint, you’ll need to update this code. We recommend never hard-coding the API endpoint and instead using the endpoint returned in the response to your initial connection to Salesforce. For example, a SOAP API login() call returns a server URL endpoint that you can use for subsequent SOAP API calls.
If I have a custom web service that I created in Salesforce that is consumed by a .Net application, the URL (in the web.config) looks something like this:
https://cs15-api.salesforce.com/services/Soap/class/My_Web_Service. Does it need to be updated? Also, note that the partner WSDL references: