I present to you Beauty And The Beast:
What you can see is Domino Designer 6.5 running in Windows XP, which is running in Parallels, which is running on my Intel-based Mac mini.
In use this setup is surprisingly like actually running Designer as an app in OS X. You can Alt-Tab in the virtual machine, which takes you to and from Windows-based browsers and Designer. You can also Opt-Tab to switch between Safari and Designer. The difference is you don't see a Domino icon while tabbing between apps. You see the Parallels logo instead. That aside is works quite well.
While I wouldn't like to work this way all day when doing Domino-intensive development it's ok for emergencies if I want to make some changes without booting a separate Windows box. If you want a Mac, but can't live without Designer, this is a viable solution.