I would think that Northwest has probably one of the worst odds at scoring International Upgrades. If you purchase a full Y ticket to Asia or Europe, the upgrades come out of a better bucket (think -- I - ) which is suppose to be similar to the Z bucket. However, if you look using a tool like ITN, you will find it is pretty rare to find any Z inventory on any days to Europe or Asia for this summer. However, some of the elite members have told me that they have indeed been able to upgrade, even in the summer, so in theory it is possible - I just think the average joe would likely not be able to upgrade well in advance on NW international flights in most cases. Less then Y fares are even harder then Y to upgrade on. Note only Y - B fares can be upgraded, except to a handfull of Asian destinations (excluding China) - you can also upgrade Y-B & M fares.

AA - I found there was some availability out there ahead of time. Especially I think from JFK-Europe. The further the distance (say LAX-LHR) the less likely you are to get the upgrade. However, with the new $500 per return upgrade fee, there may be even more seats open this summer. Also if you are looking at upgrading more then 1-2 people on the same flight, your chances will be much less on any airline.

UA - From what I have read, I believe UA is about as easy to upgrade on as AA. However not all fares are upgradable.

BA - I think is pretty good about upgrades, except they go one class above. So from coach you only go to coach+, from coach+ you go to Business, and Business to First. The best value in my opinion then is to buy a coach + (world traveler) ticket and upgrade to Business.

I am not the expert on upgrading international tickets so this is just from what I have learned from reading the forums at FT. Also note, sometimes if you are elite with a particular airline, you may have an easier time upgrading international flights.