There are a number of upcoming changes to the three main airline alliances, so I thought it would be useful to summarise. I've included known changes but excluded speculative future members - only those confirmed by the alliances to be joining are mentioned below.
|Airline||Oneworld||Sky Team||Star Alliance|
|Continental||out 24 Oct 2009||in end Oct 2009|
|Copa||out 24 Oct 2009|
|Brussels||in Oct 2009|
|Mexicana||in Oct 2009|
|TAM||in early 2010|
|Aegean||in mid 2010|
|S7 (of Russia)||in 2010|
|Air India||in 2010, maybe|
Continental has a few days in between leaving Sky Team and joining Star Alliance - it was a bit too hopeful to have a seamless transition.
Air India was originally scheduled to join Star Alliance in March 2009 but is reportedly nowhere ready (alliances have a long list of criteria which new members need to comply with).
Confirmed future alliance members sometimes become partners of some airlines within that alliance ahead of the join date. This means codeshares, mileage earning (although not necessarily status mileage), and award redemption. It doesn't usually extend to status benefits. An example of this is Brussels Airlines is already a partner of Lufthansa and Miles & More frequent flyer program.
Partnerships with non-alliance airlines also sometimes change when airlines join or leave an alliance. For example Continental ends it's OnePass partnerships with Alaska, Horizon and American Eagle on 24 October 2009.