Relevant facts:

1) Is it a violation of the GPL or copyright law to release a WordPress theme that is not licensed GPL?

Highly unlikely. Here are two great writeups by a lawyer which make the issue very clear:

http://perpetualbeta.com/release/2009/11/why-the-gpl-does-not-apply-to-premium-wordpress-themes/
http://perpetualbeta.com/release/2009/12/why-the-gplderivative-work-debate-doesnt-matter-for-wordpress-themes/

If Matt was so sure this was the case he could sue Envato. He's not, so he's banned the mention of Envato by WordCamp speakers and sponsors.

2) Pearson probably DID violate the GPL.

Pearson's theme, at the time of the original dispute, used GPL code copied and pasted verbatim from GPL WordPress theme(s)/the WordPress core package itself.