Oops! Fixed my own bug. Mail Plug-In "Attachment Tamer" (formerly Mail Attachments Iconizer) needed to be updated from version 3.0.5 to version 3.0.6. to fix "problems with attachments inserted from other applications on Mac OS X Lion". After the update, emails with and without attachments are now working again between FileMaker 10 and Mail. Sorry for the false alarm.
Interesting that installing Lion did NOT disable the Attachment Tamer plug-in like it says it should with all plug-ins.