An issue came up today where we were wondering how to put a non-secured page in a secured web site using forms authentication. After a bit of research, I found that you can add this short snipet of code to your web config...and you have it - an annonymously accessible page in a secure site.
...
<authorization>
<deny users="?"/>
</authorization>
</system.web>
<location path="enviaSMS.asmx">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>