As mentioned by others, the 5100 prints just fine using the Mac drivers, which is the same way the 3800 prints. I've been printing comparing a 5100 against my 3800 for about a week now.
Yes, to print with the 16bit plug-in requires CS3 to be running under Rosetta. This does work (I haven't used it yet, but on Friday watched a Canon rep demonstrate it on a MacBook.) It shouldn't be crashing ... perhaps you have some other plugins that are not universal binary?
I assume you enabled Rosetta by doing a Get Info on the Photoshop CS3 application file and checked the box.
The results using the Mac drivers are outstanding. The only thing I'm not sure about is if the length of print is a limit of the Mac driver. If so, you would have to use a RIP with either printer or use the plugin on the 5100.
The Canon rep said the Universal Binary driver was "any day now".