We are looking to secure a bunch of ASP.Net 2.0 .asmx web services. The which will host the web services is already forms authenticated.
Is it possible to secure the
The thing with form authentication is that its designed for people, where as a web service is designed to be consumed by a client application. While it is possible to do the authentication like this, it's the wrong way of thinking.
The level of security needed obviously depends upon the sensitivity of data that you're working with, but I'm going to assume its at least somewhat sensitive (but less than bank transactions). You could perhaps use SSL and passing a username and password as jle suggested, while I was typing this, or you could require an api key much like flickr does.
Another more secure option is to only pass the username and password once (and with the security of ssl) and have that give out a token that is valid for a period of time. This has the benefit of protecting the password information, and avoiding the constant overhead of ssl.
As mentioned though, it highly depends on HOW sensitive the information is that you're trying to secure.