Don't upgrade to OS X Lion until FileMaker Inc. releases a compatibility update to FileMaker 11.
FileMaker Pro 11 is the only version that FileMaker Inc. plans to certify for Lion. If you are using an earlier version of FileMaker (prior to 11), don't upgrade to Lion unless you see enough evidence out in the world or with your own safe experiments to determine that your FileMaker database(s) will run OK in that version of FileMaker under Lion. If you do run into a problem, you'll be on your own.
FileMaker is excited about the powerful features in OS X Lion and iOS 5. We are committed to be compatible with the latest Apple operating systems and the shipping versions of FileMaker products will be updated to support OS X Lion and iOS 5.Since FileMaker 8 is a PowerPC app, it will not run under OS X Lion at all due to the lack of support in Lion for Rosetta. FileMaker 8 and any earlier versions of FileMaker will not run under Lion.
Speculation:There's a pretty good chance that FileMaker 8.5 - 10 will work using FileMaker files on their own or as guests (with some minor bugs). There's less of a chance that you could have a machine running Lion hosting filemaker files or older versions of FileMaker Server hosting files from a Lion machine. This is how it went with Snow Leopard anyway.
People will certainly try things with Lion and experiment if they have an irresistible Lion feature they want to use. Keep an eye out for findings. I will post what I find to this blog.