Just ran in to an issue of security when having many web applications. I needed all the web application to use the same security access, after a little research I found that Client Application Services in VS2008 works a treat, as it has been written to handle security in the cloud.
I found a few good articles on the subject
Client Application Services with Visual Studio 2008
Implementing Application Security with Client Application Services in VS2008
Best of all I just love how it handles if your remote server is off line or not available
ConnectivityStatus.IsOffline = true;
To get this working in your web application you will need to make a few changes to the Web.Config, and then point the serviceUri to your service
<sectionGroup name="applicationSettings" type="System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
<section name="testing_Security.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</sectionGroup>
<appSettings>
<add key="ClientSettingsProvider.ServiceUri" value="http://localhost:55666/AppServices/Profile_JSON_AppService.axd" />
</appSettings>
<membership defaultProvider="ClientAuthenticationMembershipProvider">
<providers>
<add name="ClientAuthenticationMembershipProvider" type="System.Web.ClientServices.Providers.ClientFormsAuthenticationMembershipProvider, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" serviceUri="http://localhost:55666/AppServices/Authentication_JSON_AppService.axd" />
</providers>
</membership>
<roleManager defaultProvider="ClientRoleProvider" enabled="true">
<providers>
<add name="ClientRoleProvider" type="System.Web.ClientServices.Providers.ClientRoleProvider, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" serviceUri="http://localhost:55666/AppServices/Role_JSON_AppService.axd" cacheTimeout="86400" />
</providers>
</roleManager>