When I sold my 10D just prior to the 20D's launch, it was my every intention to replace it with the 20D.
I happened to be changing lenses at the time also and actually tried out the 1D MkII. Okay this doesn't justify it's price tag, but the feel, build quality and features simply eclipsed that of my old 10D and at that moment I wanted one.
I went away and thought about it for a while, afterall the 20D was being offered at very low introductory prices. Also if you consider the Canon consumer SLR camera market, there has been the D30, D60, 10D, 300D and now the 20D. This is a huge growth market and is prone to change more quickly. In 12-18 months time there will be a replacement for the 300D and 20D.
The professional range doesn't seem to change so frequently and I consider the EOS 1D MkII to be a camera that I will not outgrow in a hurry. I will probably never use it to it's full potential, and I will not be looking to upgrade anytime soon given my investment.
Overall I think the 1D MkII was the right choice for me.