A few users who have tried the RSVPMaker plugin report problems with getting 404 page not found errors when they try to follow a link to one of these entries. Let me know if you experience this issue, as I’m trying to track down the cause. It seems to work fine for most people, but I may be running into some kind of clash with other plugins that manipulate the WP rewrite rules object.
One website manager I’ve been working with has it functioning correctly on one site she manages, but not another. Very puzzling.
Update: 12/29/2010: I think I’ve now resolved this issue by changing some of the code that sets up the custom post type and sets the rewrite rules.