I was awarded the Registered on Launchpad trophy when I signed in. Will Accomplishments look in the Launchpad history to see that I have previously submitted bugs etc? It's been a couple days now and nothing so far. Thinking it may only register bugs reported after install.
Actually, it seems to me that you were affected by our temporary validation server lag. It was not syncing trophies properly for some time, and that's probably the issue you've met with.
Also, to answer your question: Trophies are awarded for things you have done in the past (like previously submitted bugs), as long as there is a record of such event in Launchpad (which is the case most of time).
Since it is all working now my suggestion would be to do multiple restarts with time in-between. Maybe one restart a day for a couple days. It appears that a restart is able to grab the awards better than just checking for the accomplishments.