Here's my story: I've imported my CRM 4 solution to CRM 2011. Most things look OK, plug-ins mostly work. This import process was done through the CRM Deployment Manager.
Step 2 I wanted to test building a Solution, adding all my customizations to it, export it, and import it to a second organization on the same server.
Its gotten fairly close (its a HUGE improvement over CRM 4). But its failing when it goes to load my plug-in assembly. The CRM Trace and event log aren't providing me any more help than the solution import log file:
Failure 0x80044191 Unable to load plug-in assembly.
My assembly is deployed to disk so its available. I tried unzipping the solution file, and adding my assembly in the solution hierarchy but that didn't help.
Is it because they're CRM 4 plug-ins? Must they be updated to be imported in CRM 5?
Ended up being I was referencing an assembly not in the GAC of the new server. Wasn't related to solutions per-say and I couldn't register directly with the plugin registration tool either (though that gave me more information). So if you see this I recommend trying to register the assembly using the plugin registration tool to get better information.