After a lot of tbshooting it came to me to check versions/service pack levels. Sure enough the live/main site/server was running WSS3.0 SP1, while the servers I as attempting to join were running WSS3.0 SP2. After I upgraded the live WFE site to SP2, I could then add the other servers.
*Huge* got-ya hit me though in this process. I added a server that had the RS_Sharepoint add-in installed. A few minutes after adding it the main site when offline with nasty .NET errors. Event Logs on the server quickly showed that the web.config file was trying to load the Report Services addin modules!. I can only guess that the new server did some sync/? that caused the farm to think these were part of it.
I removed this server from the farm but that didn’t seem to help. Maybe if I’d waited longer the web.config would have sync’ed differently but….maybe not too.
Still confused since on the new server I had un-installed WSS and re-installed as part of tbshooting the add-to-farm problem. Never uninstalled the rs_sharepoint add-in but I also didnt’t reinstall either. Odd that it still has its hooks into the configuration.
Fix was to install the rs_sharepoint addin on the main server so those modules referenced in web.config were available to load. I did try to edit web.config and remove but that generated 500 errors – may have edited out too much but it would have taken more trials to figure out which could/couln’t be removed. Seemed easier to just install the add-in…which has worked.