The GPL is attached in the zip that can be downloaded from the site. Above the download link it says "Released under the Gnu General Public License". He may release it under other licenses too, but that's how it is at the moment. I don't believe there is an "obligation" to share any changes under the GPL except to people you distribute the software to.
Sushubh: well, yes, it is more powerful in that respect :) Which is why I'd be more likely to use it.
Poting features from TXP to WP was an afterthought I admit, WP seems feature rich for a blog. What was specifically on my mind was taking things from WP and putting them in TXP. (Those two sentances seem to contradict eachother a little... I find that TXP has better overall structure (using sections+pages+clean title URLs brings it above a single Blog CMS) but I'd like to add specific features to that structure, things like the <!-- more --> excerpt system of WP, and Trackback. Dean says that he is more interested in making better features rather than copying features, and as he's the only official developer, my reaction is to have a go myself.
Since posting, I did have a go at moving the <!-- more --> feature, but I only managed to reuse half a line of code... but then it only needed one line of code, so that's 50% reduction in work :)
The way that WP is coded is very different from the coding in TXP. I suppose I should have expected that. I feel more at home in the TXP code, though at this stage that is probably because the is so much less of it than in WP!
(When I made the inital descision to have a look at TXP first or WP first, the main factor was that I prefer yellow to gray)