Just a quick note, that came up during last week’s push to tame the review queue: the Apache 2.0 license is GPL-compatible, but only with version 3.0 of the GPL. Works that are either GPLv2.0 or GPLv3.0 are appropriate to be hosted in the Theme directory, so that’s not a problem; however, works that incorporate or bundle Apache 2.0 works must use either unversioned GPL, or GPLv3.0 explicitly.
The most likely occurrence of this issue is with Themes developed using Twitter Bootstrap. When reviewing such Themes, please be sure to check that, if the Theme is licensed under GPL, that the license specifies either unversioned GPL, or GPLv3.0.
(And if for any reason this interpretation is incorrect, please discuss in the comments.)
Note, by “unversioned” GPL, I am referring to the current version of the GNU GPL, which can be found at this URL, and which currently is GPLv3.
What is important is the actual license text associated with the Theme: whether called simply “GPL” or explicitly GPLv3, the license text must be GPLv3. A Theme that claims the license is “GPL”, but that ships with a license.txt that is GPLv2 would not be able to bundle an Apache 2.0-licensed work.