Is it possible to secure a web service with Forms Authentication?

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 web services using forms authentication?
What are the pros and cons and other possible ways to achieve this. We certainly don’t want to pass a username/pwd or token in each web method call.

JqGrid addJSONData + ASP.NET 2.0 WS

I am a bit lost. I’ve tried to implement a solution based on JqGrid and tried to use function as datatype. I’ve setted all by the book I guess, I get WS invoked and get JSON back, I got succes on clientside in ajax call and I “bind” jqGrid using addJSONData but grid remains empty. I do not have any clue now… other “local” samples on same pages works without a problem (jsonstring …)