I too collect the Architecture line. Regarding ones getting discontinued, surprisingly so far only the 2008 John Hancock Center has been fully discontinued, with the 2008 Sears Tower rebranded and repackaged in 2011 as Willis Tower.
That being said, I'm anticipating within the next year at least the 2009 models disappearing soon. That includes the Seattle Space Needle.
The White House came out in 2010, Rockefeller and Brandenburg 2011, Big Ben and Sungnyemun 2012.
So, while Lego is free to discontinue any of them at any time, I'd say the one that has the highest chance of going soon is the Seattle Space Needle, as it will be pushing 4 years as a set in 2013.