Wow, I started the post last night before bed and get to have a good read on the packed Friday morning Tokyo rush hour train :-)
So here's my time line and reasons for upgrades (other than the "want factor")
Late2006/early2007: Bought a 2nd hand 30D. I was shooting film with a canon EOS 7 and a Mamiya 7II, I wanted to stick with film but I was getting job offers with a very quick turn around and needed digital so I dipped my toes in the water, slipped and fell into a big digital pool which I only climbed out of (slightly) last week.
2008 summer. Bought my 5D new. Things were going well workwise and I wanted full frame with better IQ, the 5D was in my price range and I went for it. I was very happy with the results. Kept the 30D as back up.
Summer 2009. Bought the 5DmkII, sold the 30D. I bought the 5DmkII as an agency I worked for deemed only photos taken in RAW on a 12 megapix camera or more could be submitted. They hinted at an increase to come, so I plumped for the 5DmkII. Another factor was I had started shooting fashion week in Tokyo and I wanted something with really good high ISO capabilities. For a while I was shooting with the 5DmkII and had my 5D as a back up.
Late 2010. Bought 7D, sold the 5D. During two consequtive wedding shoots I had problems with my 5D. The first time turning it on and off worked, the next time the shutter stopped working and I had to shoot with only the 5DmkII. I took my 5D to canon, got it fixed and decided to sell it. Got a good price for it and bought a second hand 7D that was in mint condition, so essentially I swapped bodies. I was getting married that month and really wanted the 1DmkIV but I had still to pay for the honeymoon and I felt it better to make sure I got that right, money well spent as a happy wife makes for a happy life ;-)
This week. Sold the 7D for slightly less than I bought it for and got another 5DmkII. I was really happy with the 7D, for runway shoots it was my main camera, for group shots at events I often used the built in wireless speedlite controller which was a real life saver, the AF and FPS was much better than the 5DmkII, but overall I felt the 7D wasn't so good for me in terms of IQ when I cranked it up over ISO. In that respect the 5DmkII wins hands down IMO, I shoot a lot of indoor events and I was worried that if my 5DmkII stopped during an event the IQ of the 7D wouldn't be good enough. The 5DmkII prices have dropped a lot here recently, the selling the 7D and a decent hire paid for the new body, it gives me more peace of mind so I felt it was worth it.