How to determine all the groups a user belongs to (including nested groups) in ActiveDirectory and .NET 3.5

I have an application that uses ActiveDirecotry authorisation and it has been decided that it needs to support nested AD groups, e.g.:

MAIN_AD_GROUP
     |
     |-> SUB_GROUP
              | 
              |-> User

So, the user in not directly a member of MAIN_AD_GROUP. I'd like to be able to look for the user recursively, searching the groups nested in MAIN_AD_GROUP.

The main problem is that I'm using .NET 3.5 and there is a bug in System.DirectoryServices.AccountManagement in .NET 3.5 whereby the method UserPrincipal.IsMemberOf() will not work for groups with more than 1500 users. So I can't use UserPrincipal.IsMemberOf() and no, I can't switch to .NET 4 either.

I've worked around this last problem with the following function:

private bool IsMember(Principal userPrincipal, Principal groupPrincipal)
{
    using (var groups = userPrincipal.GetGroups())
    {
        var isMember = groups.Any(g => 
            g.DistinguishedName == groupPrincipal.DistinguishedName);
        return isMember;
    }
}

But userPrincipal.GetGroups() only returns the groups of which the user is a direct member.

How can I get this to work with nested groups?