Well, it's changed a bit since then, so any solution crafted for that wouldn't even work, half my plugins are broken from changes to core. The whole field system kinda screwed up any plans for it.
Ultimately it depends what types of information you were trying to import I guess. If it's just threads, you could possibly import the first entry of each and set the old ticket number, then replay all the rest, but that would be messy and very time consuming. Likely have to modify core to accept the email timestamps..
If it's really basic: just historic ticket/user/datetime all closed, no replies, no fields, matched per topic/department, yeah, that wouldn't be terrible.
I'm seeing the value in a migration plugin now. Something to help folks move from another system to this, they are all so complex and differentiated that we'd almost need a common protocol to convert them into first (like xml/json).
What system are you trying to convert from?