There is not a .asmx service but there is a wcf service for authentication and roles, I was able to use it to authenticate from silverlight quite a while back when I was experimenting with silverlight.
The urls are
there are settings in Web.config to require ssl for these.
For documentation just google for "AuthenticationService.svc"
It has been a while since I experimented with it so I forget all the details.
Hope it helps,