Did you ever get this resolved. We are having the same issue at our company. I was told through our support that Inventor 2011 fixes the issue.
AD looked at our data and our problem was being caused by a decal in one of the parts - by changing it to an extrusion of the artwork it suddenly worked fine. Strange, i know...
whether this suggests a problem with decals and iassemblies or we had a 1 off case, dunno... Got any bmps or decals in the assembly?
(due to it working as an extrusion I have to admit I don't know if it works as a decal in 2011 - can't really justify the time to re-model this old assembly just to test at the moment)
Sam M.
Inventor and Showcase monkey
Please mark this response as "Accept as Solution" if it answers your question...
If you have found any post to be helpful, even if it's not a direct solution, then please provide that author kudos - spread that love 😄