We received confirmation of our flights we'd booked through Expedia and figured we might as well pick our seats on-line. Turns out we couldn't as we didn't book directly with Asiana Airlines (Korean company).
Not the biggest of setbacks as we still have four months to go but we've dropped a line to Expedia and they have confirmed we can only reserve seats through themselves, which we've now done.
Seat Guru is a neat website and allows you to check out schematics of the layout and also what each seat offers, ie power outlets etc.
Now all we have to do is wait to see if it gets done. And also if they can reserve a vegetarian meal, advise on baggage allowances and tell us how to check in. Have to say, if we'd known about this, we'd have booked directly...