Since the beginnings of ASP.NET 1.1, Microsoft disabled HTTP GET and POST actions on .NET Framework based web services (for security reasons). If you want to enable these protocols, add the following XML to the the system.web node in the web.config file:
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
I recommend that you only enable these protocols if your service does not care about security. In my case, I wanted to expose an already public non-rpc service as a web service (with some business logic thrown in) and still enable legacy clients to use this new service without the SOAP overhead.