Quite by accident, I just noticed that a number of Apple-supplied Dashboard widgets on my MacBook were running under Rosetta. Specifically, Flight Tracker, People, Phone Book, Translation, and Unit Converter; the others with plugins had universal binaries.
I did use the Migration Assistant to preserve everything from my old PowerBook, but that shouldn’t have overwritten system-supplied widgets that were already present on the target machine. But maybe it did. Or else my MacBook shipped with some PowerPC cruft that hasn’t been caught in the last four OS updates.
An interesting note is that the PowerPC-only version of the Unit Converter widget is only localized for English and Japanese, while the universal version adds about a dozen more, despite the Info.plist file claiming that they’re both version 1.2.1.