... Until, that is, I kept getting the error, Unable to find XXXX.Membership in System.Web....
There's all kinds of references of how to use the default membership provider in a winform or console app, but not much on a custom membership provider. After a good bit of digging, I found this blog: http://geekswithblogs.net/Shadowin/archive/2007/04/05/110864.aspx
I failed to put ", AssemblyName" in the reference in the config. So it was defaulting to System.Web.
Your entry for the custom membership provider should look something like this:
< add name="AspNetSqlMembershipProvider"
connectionstringname="SiteSqlServer" / >