I've run into this as well in a project with hours and hours of legacy footage. Started having the Bad Video Frame issue and came here and read the posts...
So after reading the posts on here are these really my only two options:
1. Do a complete clean reinstall of Snow Leopard, reinstall Final Cut Pro, trash any old preferences. Or is there any easier way of doing this?
2. Buy Lion and buy the new version of Media 100 2.1.
Do I have any other choices? Is there any way to XML over to FCP any of these old legacy projects and open them there - doesn't look like it as you can't really open them at all. Can I really not run Media 100 v.1.5.1 off of Lion?