Sharing a list across two seperate collections

[M.@discussions.microsoft.com Bill M.]

I have an odd requirement and I was hoping to get some suggestions.

I have two seperate MOSS installations. On one I have a list, calendar
specifically, and customized web part that consumes it. The other
installation wants to have that web part on their site and have it hit the
same list. The installations are in different domains as well.

My first thought is to use the built in web services to access the list, but
some have performance concerns and I've been tasked with finding out if
there're any other possibilities.

TIA