Warning: Cannot modify header information - headers already sent by....

After installing the new files and then uploading the old wp-config.php file, I was beating my head into the wall. 

Finally, after debugging through things, I found that the WP 2.5 wp-config file has an extra newline character (’\n’) at the end of the %> tag.  Completely undetectable, I lost hours trying to solve the problem.  It appears that 2.5 was either more forgiving or there was some extra code in 2.6 that caused a problem with the extra newline.

I’m just posting this just in case anyone else runs into trouble – check the end of the wp-config.php file for any extra newline characters.