I just purchased Minecraft yesterday and unfortunately ran into the 'Pixel Format Not accelerated' error. I'm currently using a Radeon x1900GT as my 4870x2 went bad and I'm waiting for the next big thing (Antilles). Anway, I checked my opengl version with  Open Gl Extensions Viewer and saw that my version was 1.1. Upon checking Wikipedia I noticed that my card (as well as a lot of other legacy Radeon's) supported 1.4 and higher (1900gt is Opengl ver 2.1). So, after checking around on different websites I decided to try to re-install my ATI drivers.

I used the latest possible driver (Catalyst 10.2) and found out my Opengl version was still at 1.1. I figured it had to be the drivers so I tried to un-install them (rarely works right) and install the previous 9.12 set. After fooling with that a few times and finding out it wasn't going to work I decided to trick the Cat 10.2 into installing the 9.12 drivers.

I started a custom install of the Cat. 10.2 and when it got to the 'options' menu I went into Windows Explorer and navigated to the 9.12 drivers that were unpacked on the harddrive. (C:\ATI\Support\9-12_vista64_win7_64_dd_ccc_wdm_enu\Driver\Packages\Drivers\Display\W76A_INF\B_92022)

I copied all of them into the the same folder in the 10.2 unpacked driver folder, over-writing the supposed newer files. Then I finished my installation and fired up 'Open Gl Extensions Viewer' and saw that I finally got my opengl 2.1! Fired up Minecraft peice of cake..works like a charm. :D

SO, in a nutshell the Catalyst 10.2 drivers must have an old version of opengl in it. So use 9.12!

Hope this helps someone.