I'm a bit late getting onto this thread, but the earlier posts re the op-up "algorithm" are interesting. From personal experience (not conclusive, but certainly suggestive), I think the "pax value" idea si most likely correct. I've been at WP since 2003, but was mostly flying discount Y - op-ups were virtually non-existent. For the last 4 years, I've been doing mostly int J, with some dom Y, for a spend with QF of between 100k and 150k per annum. On the 6 occasions where I've done int Y (because the travel has not been funded by our customer or I've chosen to have a break and come home in the middle of an assignment), I've had 4 op-ups (SYD-LAX-SYD and SYD-LHR-SYD), which I think lends support to the "value" theory. In the same timeframe, I've also had several dom op-ups (mostly BNE-PER or PER-BNE)
The long-haul upgrades were nice, but the best op-up I've received happened a couple of months ago BNE-PER. I was travelling with my boss, who tried for an ODU in the QP about 30 mins before scheduled boarding, but was knocked back (no seats). At the boarding gate, I got the red light, and was then handed a BP for 1C. Needless to say, the boss was NOT impressed - he whinged about it for the entire week we were in Perth.