You need to copy a few things over from your working Tiger installation. With all these in place Interface Builder 2.5 starts nicely and doesn't complain. I haven't tested it more.
From /System/Library/Frameworks (Tiger) to /Library/Frameworks (Leopard) a different target directory!
(Tiger) to /Library/Frameworks
(Leopard) a different target directory!
From /Library/Application Support/Apple/Developer Tools
(Tiger) to /Library/Application Support/Apple/Developer Tools
(Tiger) to /Developer
- IB Plugins if available
(Tiger) to /Developer/Extras