lammert - 5:38 am on Jul 21, 2010 (gmt 0)
Not being a GPL expert, but as far as my interpretation goes, this is one of the reasons the LGPL license was invented. The Lesser GPL license is for situations where a piece of software is used in another piece of software, but without the requirement that that new software needs to be published under the GPL. LGPL is mainly used for libraries and other back-end software. The LGPL license allows other software to use these libraries and back-ends without the viral rule that the same license should be applied to the front-end application.
In my opinion, WordPress can be seen as a code library/back-end which--if removed from the Thesis theme--makes the Thesis theme worthless. The function of the WordPress code base is therefore that it is a crucial library for the functioning of Thesis, and Thesis should be released under the GPL.
The only exception is when WordPress would have been LGPL licensed, but that isn't the case.