No, not at all.
If I have had good experience with a particular company, I will tend to look at that company first when considering another purchase. But I will always comparison shop to see who currently has the best product at the best price.
I was a big fan of AMD with the Athlon and Athlon XP series, and I'm rooting for them to do well with the 64. But if I had bought a new PC 6 months ago, I would have been getting a P4.
I was alsow a big NVidia fan.
But I would definitely buy ATI if I was upgrading right now.