Single sign-on

Hello,

I'm trying to implement single sign-on functionality across websites on
two different domains.

Site #1 is running shrink-wrapped software with a member database which
we have no access to and don't really want (haven't found an API,
either). We're going to receive a nightly data update to keep our
databases synchronized.

Site #2 is ours, an ASP.NET application we have full control over, and
on a different domain.

So, a user would sign in to Site #1, and seamlessly follow a "link"
(posted form, or some other option) to Site #2 without having to log in
again.

What options are there for accomplishing this?

I've read a ton of articles and newsgroup posts on different schemes
but haven't found one that quite matches our situation yet (different
domains, possibly different scripting environments, and limited control
over Site #1). Unfortunately I've run into a brick wall on this one. :/

Any ideas?

In case it helps others, some of the links I found are as follows:

http://www.eggheadcafe.com/articles/20030701.asp
http://www.codeproject.com/aspnet/PassThroughSecurity.asp
http://www.eggheadcafe.com/community/aspnet/6/84770/single-sign-on-ss0.aspx

.